For consistency's sake, it's best to reroll all if they roll too many. If they roll too few, I'm fine with them adding dice to get to the right total. In that rare corner case where it's obvious that they succeeded on the roll easily (e.g., the 4 evades scenario), then I'll often just accept it. It seems EXTREMELY douchey to try to argue about having more/less dice rolled will affect the outcome and making them reroll them all. That being said, those corner cases are extremely rare in being that obvious, so rerolling them all is the better solution.
Much more frequent is disputes over what is or is not a cocked dice, particularly when only slightly cocked (like on an asteroid marker). Far too many opponents try to let successes lie and reroll failures which is pretty much the definition of cheating.