You know how weapons deal dice instead of a set number. That's because sometimes they hit something vital, and sometimes not so much. So assume that a groin shot to a man is rolling high on a normal damage roll.
Agreed (unless its a monster that's statted for it, like how some have sub-locations for tentacles (TENTACLES, I said TENTACLES!) and stuff).
That being said, I will let a PC try to attack any part they want, but that will only affect the flavor text of the description, not provide any other effects or bonuses.