Many will call it a common courtesy, but I for one, am not sending out thank-you letters. It is certainly not mandatory.
I thanked the admissions staff, interviewers, and students while I was there, and I don't feel it necessary to thank them again. After all, I paid lots of money to interview at their schools, and in quite a logical sense, they should be thanking me (since I'll also be paying tons of $$ to attend one of their schools).
But alas, if you choose to do so, then go for it! I can't see a thank-you letter actually bolstering your application, but it certainly can't hurt either. In either case, the thank-you should be addressed, or sent, to the person(s) whom interviewed you.