"..does (dead square case mouths) even really matter when it comes to accuracy of the round?"
No, not for most of us. It's believed the case mouth will be expanded hard against the chamber before a bullet even clears the mouth so any tiny lenght difference between one side and the other of a neck is virtually meaningless. That's certainly true for factory rifles and chambers and common cases. We will never get two consecutive groups in the ones as are sought by BR guys and their loading methods are rarely of any use to us. In fact, some of their methods are actually harmful to factory rifle accuracy!
IF you reallly want to learn if one side of a trimmed neck is longer than the other check it with your dial caliper, it will show up if it's measurable. I've used both the Lyman Universal and Accu-trimmers for decades, neither is BR quality but neither are my rifles, or me, so what difference would it make to my ammo? Zip! (My best consistant five shot groups from my best rig run in the 3/8 - 1/2 MOA range.)