It's more in than on. And I have a couple of thoughts on the issue. First one is, if rings are a problem, wear barbells.

The second one is more complex. I don't think I run into things with my boobs all that often. And I'm most often wearing something over my chest... a night shirt, a bra, a parka... something. I just don't see getting nipple rings caught in things to be a realistic problem.

I had a dear friend who was an older woman, who died a couple of years ago of AIDS. She had her nipples pierced, and often wore a little chain between them, with bells on it. It amused her when people looked around for the source of the ringing.