I would strongly disagree with anyone that says it is not important to send thank-you notes after your interview. Personally, I sent hand written notes to each of the people that I interviewed with and tried to send one to the secretary/program coordinator that assisted in organizing the interview day. If I felt like one or two of the residents took a particular interest in trying to be available or helpful I sent one to him/her. Make it personal, don't kiss up, and it can be as simple as thanking them for taking the time to interview you and introducing you to the program.
Did it matter? Yes. My interviews resulted in about 150 hand written thank-you notes, but what I can say is that I also matched at my first choice for ophtho residency and for transitional year (both pretty competitive). Before I left my transitional year, we were allowed to look at our file. They still had my thank you notes in the file for the program director as well as the coordinator, and on my interview data sheet there were "extra points" for having sent the notes. When I interviewed incoming 4th years for that transitional program, it was noted whether they had sent at least one note or if you did not send a note. I can guarantee it hurt several people that did not send any notes and it helped those that sent notes to the residents. For my ophtho residency, I sent notes to all of the interviewers, the secretary and two residents. Once I started residency, I found out that the resident I sent a thank-you letter to went to the program director and let him know. He made it known that he thought I would fit well in the program, and I think it probably helped.
Common courtesy goes a long way these days. Remember, some of the people that you will interview with are from your parents generation and it is expected. I know several young faculty that feel the same way. Don't think that not sending a letter can't hurt you. Yes, it takes time to hand write 5-10 letters each night after an interveiw, but it will be worth it. Programs are looking for residents that are willing to go the extra mile. Even if it does not help you rank higher, it will start you out on the right foot at the program you eventually match with.