As someone who RECEIVES your thank-you notes, I have to say that, at least at the school where I interview, the admissions committee doesn't even see the thank you--it goes straight to the interviewer, who generally does not sit on the adcom, just makes recommendations to them. By the time your letter gets to us, our recommendations have long since been turned in. On the other hand, out of the 25+ applicants I interviewed this season, only 2 (!!!) were courteous enough to send me thank-yous. In one case, the note confirmed that I had made absolutely the right decision in whole-heartedly recommending acceptance for the applicant, and in the other, an applicant I had been less than impressed with during the interview, I felt better about having made the recommendation that the canditate be allowed a second opportunity to interview, with someone else, since there just seemed to be a lack of "chemistry" between us, rather than any problem with the person as a viable candidate for acceptance. I don't know, however, if that actually occurred.
I would send the thank-yous unless specifically instructed otherwise--it can't hurt, and might help. For residency, I sent the same type-written letter (individualized for each program) to everyone I interviewed with, plus the program director, and then also enclosed a personalized, hand-written note on stationary cards embossed with my name.