I know that in Shorinryu at least a slight tilt backwards on the front snap kick (mae geri) is permissable, but personally I don't like it. I prefer to actually lean FORWARD if anything. Hell, I figure that I'm following that kick in, anyway. To me, the kick is like artillery; it weakens the defense and allows the infantry (my hands) to finish the attack.
Aside from keeping your guard up, any energy expended should be in your lower body. The upper body may appear to sag, since it's so relaxed.