I don't restrict nose-picking. As a matter of fact, I pick mine frequently--
-- and they see me do it, so I really wouldn't have much of a leg to stand on, if I decided to make it an issue. What I do is to let them know, clearly and often, that most people think it's gross, that they will get a bad reaction if they do it in public, and that probably it's best to restrict it to at home "en famille," or in private places like bathroom stalls. That way they know. I also make sure they realize that over-enthusiastic picking leads to sores and nosebleeds. Again, so they know-- if they want to give themselves a raw nose, that's their choice, but they can't say I didn't warn them. And they get me as a model-- I'll do it frequently, at home, but they will never, ever see me do it in front of anybody else.
So far it's worked-- DD1 goes to school, and has a zillion friends, and she knows not to pick except privately at home. She seems to have a balanced view of the whole issue. I'm happy with that.