Bottom teeth are just fine because they're covered by the tongue. I don't understand how the top teeth stay out of the way, but apparently they do (my LO doesn't have any yet).
Sometimes if they get bored or distracted, or are in pain, they may want to use your nipple as a teething toy. It has only happened to me twice, but it's a good idea to watch for signs of changing their latch, especially when the tooth is brand new, so you can take them off.