When I was interviewing a few years ago, I asked some of the students at the school I'm at now if they wrote thank you notes. They all shrugged and said no. I figured it didn't affect them, so why go through the trouble? And it didn't hurt me at all.
Recently this all came up again for residency interviews, and what I heard from the internal medicine residency director here is that any note that you send, even if you send it to the person's office, goes straight to your file. So if you write notes to two or three interviewers that say the same thing with a different name, everyone sees them when they pull out your stuff again at the committee meeting. And then you just look silly.
I didn't write notes for residency interviews. I doubt it will hurt me -- hope not. I did write to the program directors at my top few choices though to let them know I was going to rank them high.
As an aside, I'm a believer in writing thank you notes for personal things (staying at someone's house, etc). These giant, factory processes like applying for med school aren't personal enough to warrant it. If I were applying in a very small field, like ENT surgery or neurosurgery, I might write thank you notes, because a LARGE program for those specialties takes 4 people.
