Commitees yes, but the dept chair and/or PD can override or veto anything. Ultimately, they are the final determinants.
And I hate thank you letters, I don't even open them. Emails I can tolerate.... sometimes
I hope that you tell your applicants not to write you thank you notes; if not, this is sort of messed up as a philosophy.
As an applicant going through the process right now, I have
no desire to write thank you notes, but because so many other people are doing it, I feel obliged to "play the game" and write them. It takes a lot of time and effort, and, quite frankly, when you're traveling from big city to big city, the last thing you want to do is be searching for a freakin' post office or mailbox to stick them in.
Anyway, if I ever get to the level of being someone who is in the position to receive a thank you note, I am going to staunchly and adamantly insist that my interviewees DO NOT write me any sort of thank you. It's enough of a thank you for you to take a day out of your ridiculously busy schedule to come check out my program and to sweat a day of interviewing for it, too. That's my two cents.
That said, I'm still going to begrudgingly continue to send thank you cards to all of the PDs, chairmen, interviewers, program coordinators, janitors, secretaries, security guards, OTs, PTs, and candy stripers that I meet on each interview day, because everyone else is doing it too and I would be remiss if I missed the opportunity to go to a program because a residency coordinator didn't get a card with a kitten on the front with a little voicebox saying "thanks."