I go with no, for the most part.
Might should only modify in cases where you're actually able to leverage it, and the cases where it modifies Fists should be pretty rare. I see this aspect of the power relating more to the ability to use strength as a boost to Athletics for climbing/jumping/etc. or Intimidation rolls where you show off your strength (the scene where Thomas bends two dumbbells around each other would be this), or Survival where you wrestle an angry rottweiler to the ground and pin it there.
The extra power in your punches is modelled by the +X stress per level of Strength power to attacks using muscular force. After all, high Fists doesn't really mean that you're incredibly strong; it just means that you're really good at hitting things. All the strength in the world isn't going to do you any good if you can't land the blow.
EDIT: Think of it this way; how often do you have Might modify Fists without Strength powers being involved? That's how often it should modify after Strength too.