I'm with those suggesting you have a chat with your players about narrative systems. There are some games out there meant to be more like simulations of reality, but Edge of the Empire (and the other FFG Star Wars games) isn't among them. This is something I've had a little trouble with from time to time, because I was brought up on more simulationist-style games, but I'm mostly over it. I hope. (I better be... I start a new campaign this coming Friday! *chuckles*)
I like Doc's way of thinking - let shots that do extra damage (by virtue of more successes) be head-shots and the like - because it gives a nod to my more simulationist roots.