If you don't allow nipping and give them heck for it then they don't become nippy. The only time they become nippy is if you allow it. I have hand fed all of my horses treats, they have learned not to nip because they get in big trouble for it. Even a baby will learn not to nip for treats. I have raised babies from the get go that learned very early on that nipping and lipping is NOT allowed.
If you never give the horse a chance to make the mistake of nipping (and then correct them) then they will never learn that it isn't allowed because they are never told not to. Personally I would rather know that my horse isn't a nipper because I have trained them not to be, instead of refraining from something because I'm afraid of the outcome of the action.