DISCLAIMER:
Separate your rats if you are unsure of their behaviors.
This information is for educational purposes only and cannot be used to diagnose whether two rats might erupt in aggressive behavior, and cannot be used to prevent injury.
Seek help to evaluate your rat's behaviors and potential for aggression.
You have everything to gain by separating rats. You have everything to lose if you don't separate them: One rat can slice open the insides of another rat faster than you can blink. Please please keep your rats safe.

Most of the time this issue comes up is during an introduction process.

My speculation is that the people who DO follow this rationale successfully, are able to do so because they have extensive experience reading rat body language, so that the rats they allow together, are already close to being friendly. I'm pretty sure that if you showed two rats with body language saying “fight is coming”, that person might say, “Oh no, I wouldn't put those two rats together - not right now”. Suggesting “No blood, no foul” to someone with limited rat experience is at best unhelpful and at worst, dangerous.

A common conclusion some people draw is that whatever wrestling, pouncing, dragging, squeaking, boxing, sidling, that you see your rats do - even if it is very intense - even if it resembles all-out aggression - all of it is okay unless you see blood. They say, “No blood, no foul”. This rule tends to be applied to all rats placed togther, no matter what the circumstances.

I respectfully disagree and feel there are several reasons to not allow any and all behaviors - especially during an introduction process.

The reasons not to follow the “No blood, no foul.” reasoning during introductions:

“No Blood, No Foul” dictates that you should let rats interact only any way they want, no matter how intense, unless and until blood is drawn. Then, separate them. My feeling is, I one of my rats to be injured. I never want blood drawn on my sweet rats. I do not want a wounded rat before I decide to do something about their behavior.
One rat can slice open the intestines of another rat faster than you can blink. You won’t see it happen.
Rats are extremely territorial, and when we introduce them to each other, we don’t do it like they would with each other in the wild. Aggression between wild rats is not the norm. (Dominance is a different thing and is not the same as aggression. More about this topic below.) Wild rats don’t often run into stranger-rats; they aren’t suddenly grouped with strange rats who they must live with; they have vast space so that they can keep their distance from a stranger; they choose when and how to approach a strange rat.

On the other hand, we set up their captive world all wrong: We plunk them down next to strangers and expect them to feel friendly feelings. Our cages are too small, our neutral areas are small, our sessions are timed according to our schedule, not the rats’ feelings or needs. Everything about an introduction IS artificial and stressful even if we go slowly and patiently and in tiny baby steps. An introduction process is just the situation to trigger blood-letting aggression. I know this, so I feel a responsibility to be prepared to prevent all aggression before it ever takes place.
During an intro session, if one rat draws blood on another, they build up bad, negative feelings about each other. “Do Not Like YOU!”. For good reason - that bite HURT. Then we humans have to figure out how to help the rats un-do those negative feelings. This is harder to do than taking the extra time needed to insure a safe, calm, introduction process from the start.
Aggressive behaviors beget aggressive behaviors. If we allow ongoing severe fighting - even if there is no blood drawn - the rats’ aggressive behaviors will get stronger and that much harder to un-do.
On the other hand, calm mind, and peaceful, good behaviors, are more useful for showing one rat that another rat can be trusted. This is based in behavioral science.

If you read this far, thank you so much! - and you can stop here. I hope this is enough to help you feel good about protecting your sweet rats during an introduction process, and to NOT be seduced by “No blood, no foul”. And, if your interest is piqued, there is more to talk about - see below.

How or when might “No blood, no foul”, be accurate?

Babies or bonded cagemates playing together. Play can get very intense and aggressive-looking and be completely normal. We humans might be horrified at what we see and truly believe the rats have gone overboard into aggression. Sometimes I will interrupt play with “Hey, guys what are you doing?”. Two rats will stop and look at me with a very calm, “Did you need something? We’re in the middle of playing here”, and then resume their wrestling.
Most NON-PLAY behaviors that look like aggression (of rats in an established colony) are not aggression the way that term is commonly understood. Rats do not just fight each other for fighting’s sake. Resources are at the heart: Rats seek to gain access to and control over valued or scarce resources such as food, toys, treats, the best hammock. Any object in a rat’s environment may be highly valued or scarce.

Here's a way to visualize Dominance: Two rats walk into a restaurant and head for the best table. Which rat will get it? They each do something and then maybe another something, and in the end one rat will get the table (be dominant), and one will give it up (submit). This social competition over resource is what defines dominance and submission.

So just what are those, looks-like-fighting behaviors of rats - but they aren't aggression? - in an established colony?

All the social behaviors in an established colony are grouped together as agonistic behaviors. Ethologist (and Hero Rat, Gambian Pouched Rat, scientist) Roger Abrantes explains:

“Agonistic behavior includes all forms of intraspecific behavior related to aggression, fear, threat, fight or flight, ... when competing for resources. It explicitly includes behaviors such as dominant behavior, submissive behavior, flight, pacifying, and conciliation, which are functionally and physiologically interrelated with aggressive behavior, yet fall outside the narrow definition of aggressive behavior. It excludes predatory behavior.” [Emphasis added.]

On the concept of dominance, Abrantes explains:

“Dominant behavior is ... behavior displayed by an individual with the function of gaining or maintaining temporary access to a particular resource on a particular occasion, versus a particular opponent, without either party incurring injury. If any of the parties incur injury, then the behavior is aggressive and not dominant. Its ... characteristics range from slightly self-confident to overtly assertive.”

“Dominant behavior is situational, individual and resource related. One individual displaying dominant behavior in one specific situation does not necessarily show it on another occasion toward another individual, or toward the same individual in another situation.”

“Dominant behavior is particularly important for social animals that need to cohabit and cooperate to survive. Therefore, a social strategy evolved with the function of dealing with competition among mates [or cage-,vor mischief-, or colony-mates - in the case of domesticated rats - added by Gwen], which caused the least disadvantages.”

Some additional gems from Abrantes:

“Persistent dominant or submissive behavior from the same individuals toward the same other individuals may or may not result in a temporary hierarchy of a certain configuration, depending on species, social organization and environmental circumstances. In stable groups confined to a defined territory, temporary hierarchies will develop more readily.... Hierarchies are not necessarily linear, although in small groups and over time, non-linear hierarchies seem to have a tendency to become more linear.”

“Dominance and submission are beautiful mechanisms from an evolutionary point of view. They enable (social) animals to live together and survive until they reproduce and pass their (dominant and submissive behavior) genes to the next generation. Without these mechanisms, we wouldn’t have social animals such as humans, chimpanzees, wolves and dogs to name just a few.”

Need I add, and rats!
If I can tempt you to read more on these subjects, here are some References, including more than the sources quoted above:

Roger Abrantes:

Dominance—Making Sense of the Nonsense , December 11, 2011.

Pacifying Behavior—Origin, Function and Evolution, June 6, 2012.

The Spectrum of Behavior , September 9, 2011.

Patricia McConnell:

The Concept Formerly Described as “Dominance”, March 31, 2010.

“Dominance” Mythologies, Suzanne Hetts, April 7, 2010.

Dog Training and the “D” Word, April 14, 2010.

The “D word” and Social Relationships in Dogs, April 5, 2010.

Dogs & “Dominance” –What’s a Person to Do?, April 7, 2010.

Action/Reaction The temptations of the dominance fallacy, April 7, 2010.