I think that there are probably only a few places where that is an issue. As an upper level resident, I could care less who sends a thank you note, dresses properly, etc... All I care about is whether you are going to be a hard working person who I can get along with. While most programs may change their list a little bit, it is difficult to get residency interview committees together to go over a residency list every time a candidate calls and says he/she is interested in the program. This is certainly a very important part of any residency, but if you have been in practice for 5 to 25 years like many attendings have been, this process is treated like any other requirement. They show up, interview, carefully and systematically review prospective candidates and rank them, and then move on to their clinical work. Unlike most students who have all the time in the world (including me a few years ago), attendings are busy and don't sit around reshuffling the list all day. I am not saying that you have to shoot an email to the PD or drop a card at the end of an interview, I am instead trying to tell you that your letter is a nice gesture that holds absolutely no bearing on your future. Other than a few places that like to have their asses kissed, focus more on the interview and less on the stuff afterwards. If you leave a good impression at the interview, the rest of the stuff will take care of itself.