Ditto. I never wrote one of those post-Christmas-like thank-you cards and snail-mailed it like you might to your grandmother. I did send a few follow-up emails, generally to someone I thought I clicked with or just to the program coordinator/whoever I had discussed setting up the interview with the most.
On the other side, I believe once or twice(?) we got a snail-mail card from an interviewee. It wasn't terribly cheesy per se, just a little unusual. Even as a resident myself, I got several emails back from applicants, which I generally forwarded to the program director as an FYI (depending on the nature of it - I also assumed a certain degree of privacy with such communications).
At any rate, I agree that sending a "Thank you" isn't required/expected, but some sort of follow-up communication is still probably a good idea - if you can do it without sounding forced, overly formal, or cheesy, all of which are rather subjective.