I use my earring and people tend to be a grossed out. I know when I went to Turkey we were told it was really rude and we had to put our hand to our mouths. I do tend to try to cover my mouth though as it does look gross!
A colleague sits with one in his mouth for hours after lunch, picking away and twiddling it, holding it in his mouth like a cigarette. He can't grasp that it really is revolting, and I don't want to have to talk to him while he's flicking bits of his lunch out from between his teeth! Euuuugh! Maybe if he hid it it would be slightly less gross, or at least I could ignore it rather than being constantly faced with it.