How about this as a concept. It is personally what I ascribe to.
Guy approaching, but I can not yet tell if abad guy, good guy or just a guy, yet out of both our ranges (lot of squish in this I know, range being what it is with various weapons etc.), but this is a philosophical discussion so roll with it for a few sentences so I can get it out before objections fly).
If I can identify the combative range/distance/mai ai, before the unknown gets close to that range I engage him vocally by challenge to determine intention, i.e. "What's up mate?" or "How are you?" or "Can I help you?" depending on where and who and whatever. If they don't react to my statement and continue the approach, I adjust posture to change to aligning on potential threat(s). The next vocal challenge is direct, "No closer," or whatever. They continue to close after the direct warning and I initiate and do not wait for them to choose the engagement distance and time.
Of course, the above assumes that I note the approach, but that is part of the training is it not. Contrary to the whack 'em in the head with blunt object and just stop and walk away while they recover, if I have to engage a guy and he goes down, I keep him down and wait for the cops -- or, as has happened, he goes down and I leave his oncoming mates at top speed.
I tend to agree with the crowd of folks above who really don't want to wait for the other to actually swing first. Or stab first. Or shoot first. Your odds of getting messed up are just way too high and I like my skin without additional holes, marks or bruises. Definitely like my wife & daughter's that way, too.
This being said, you Must stop when the threat is ended. Period. I know more than one guy who did go explain it in court, and to a one, they agree that it was a bad decision at the time.