I feed treats for trust, but only to horses that aren't aggressive with their lips already. I wouldn't expect that offering treats will teach respect, though.
Kitten_Val, I think it depends completely on the horse. I've got three mustangs right now, and none of them are biters. The last one, however, is a horse that ought never to have been fed from the hand. Then again, she had novice owners who didn't know how to tell her 'no'. She was aggressive as all get out when we got her, but a ***** cat when she left.
The other three? One is terribly head shy and treats from the hand are a long, slow process to coax her into allowing us to handle her face without fear. I doubt she'll ever be a biter. Quiet Storm had no clue how to eat a treat from our hand when we offered it last month; it was the first time we'd done that in the year we'd owned her. She's the one I'd expect could develop a nibbling habit if one were to feed her every day from the hand. Jet is an oral horse, but not a nibblely horse. We play with her lips a lot because she appears to enjoy it. Hey, what ever it takes to get them to like us, right? But she's not a pushy personality, so I'm not terribly worried