Depends on the school. NYU only gives out the e-mails of the interviewers, so I sent them an e-mail. Michigan has 10 interviewers so I didn't send anything. Other schools, I might send an e-mail or a thank you card, depending on what they give you.
I think it's good either way. They will read all e-mails and letters, they aren't going to ignore your e-mail, they will get it. It's a nice gesture.