When going heavy, I only go a little below 90 degrees. When I'm stretching (light weight) I'll tap my chest.
People tell me I'm cheating. I watch others bounce it off their chest. (You aren't supposed to depress a chest that much during CPR.) I stop the weight about 1-2" above my chest and press. I'd rather do that with good form (timing, control) than most of the slop I see there from those that say I'm cheating.