I also own a rather mouthy horse. I had the advantage of knowing his original trainer, and knowing she smacked him whenever he'd try to groom her, so I let him lick on me for several months. Then, he bit me (not hard, not bad, didn't leave a bruise) and he pulled away and did the 'self-reprimand' act. I let this go on a little longer, but then as he continued to nip me occasionally, I decided I'd had enough. For 2 years I reprimanded at first for nipping, and when that didn't work any mouthing. Well... then I just decided that I didn't really care. He's a lovely, respectful boy besides that (also exceptionally intelligent imo), so this past year I let him mouth, nibble, whatever. He no longer reprimands himself, and his 'nips' happen very rarely now (once a month? I'm with him about 6-8 hours a week) and they never hurt above a pinch.
I'll say it depends on the horse... in my case, I don't think it does any harm. It really hinges on A) it's dangerous and B) you mind. If you don't like it, or he ever draws blood, he doesn't get to do it. *shrug*
Hope this works out for ya'. :)
"He can pitch a fit all he wants to... but he's going to do it at the far end of the leadrope." ~Eric Clark, NH Trainer
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