Even if the premise were true, which I doubt, good luck on convincing a jury of that.
If there are eyewitnesses who said you threw the first punch, then you would have the burden of proof to demonstrate not only that your premise is true, but also that this subtle movement took place.
The jury would hear from other witnesses that you threw the first punch.
IANAL, so this is all speculation.
“Assume the attitude of a person about to fight” is one of the first definitions of square up and I’m flabbergasted some of you have never heard it used that way before. In most states, they use a reasonable person standard when it comes to whether the use of force is justified. i.e. Would a reasonable person be justified in using violence to defend themselves against an attack? If someone squares up or puts up their dukes, it’s not unreasonable for them to believe an attack is imminent and take appropriate action.
I’ve heard that usage, too. But i literally square up 6-7 times a week (in a square dance) so that’s what comes to mind first. 
I’ve heard of boxers squaring off, i think. But that’s formal, and they both do it, it’s not a unilateral thing.
I’m pretty dubious the courts would recognize “squaring up” to attack as a general concept, although you might be able to convince a jury that you were under imminent threat, which is a more general defense.
Dictionary. Com gives that as the 5th of six definitions.
And the examples it gives are more metaphorical, and it says they mean “square off”.
You’d need to convince the jury not only that the other person did square up, but that you didn’t square up first. Which would be a very difficult thing to prove, given that you punched first, and therefore were presumably ready to punch first.
We just had this thread on “squaring up.”
Incidentally, if I type “definite square up” in Google, the fight posture definition is the first one given. (From Oxford Dictionaries.) I was a little surprised to find out it had an older etymology than “square up” meaning to settle debts.
Googling around, a number of lawyers’ blogs give things such as this.
My bolding.
Seeing the other person getting ready to punch someone could be seen as an imminent threat, but I think the problem is that if there has been mutual “yelling and swearing and threats” then it’s going to be a lot harder to prove one was acting in self defense.
Yes, I’ve seen “squaring off” as a term related to preparing to fight.
The problem that I see is that “assuming the stance” could be seen by some as a defensive posture. Maybe my lack of fight experience is the reason, but I would have trouble in many cases distinguishing a defensive stance from an offensive one. In martial arts, aren’t they both the same thing? (foot placement, slightly sideways, one arm forward of the other, both bent… etc.)