While it may not be essential to your acceptance it certainly reflects well upon you. I sent real "snail mail" thank yous instead of emails. I don't know if that matters at all but I think it is a lot more personal than an email, especially if you had a really good interview. As far as emailing, just call the schools and they should be able to give them to you, only do that though if you can't find it online anywhere (nothing says "I'm too lazy and want the easy way out" than calling for email addresses when they are right there online). If going the snail mail route just address it to the central office of which ever dental school they are located and send them there with the interviewers name above the normal address. Hope this helps.