I'm administrivially challenged. I hate thank-you notes, because it's more useless paperwork. I always send them when applying for jobs, because it's the thing to do, but once I found out a lot of med school applicants don't send them, I decided to cut down on my stress and not do it. Of course, I thanked all my interviewers profusely at the time.
I did send one thank-you note. That school was my sole post-interview rejection (so far; I still have one decision pending). All the places I didn't send notes, I was either accepted or waitlisted. Conclusion: they must not care that much.