bottom line, a kill is a kill. In bowling (I bowl on leagues), it doesn't matter at the end of the game if a strike was a perfect hook into the pocket the exploded the pins, or a off pocket hit that you got lucky with some off-the-wall pin action - it's still a strike.
I always prefer to headshot, if I can get it, but if all I can see is your leg through a stack of crates or under an overhang (like several points on Dustbowl), and you stand still long enough, you're still dead.