Keep in mind I am a IMG who is 2 years out and mediocre USMLE scores.
I am sending handwritten notes to everyone : residents I interviewed with, faculty, program coordinator, and director. At one interview several residents kidnapped me for a lunch - they were not a formal part of the interview. One was a former grad of my school (and is now chief resident) and the other was a woman I met at the dinner the night before and who gave a tour of the town and her house showing what could be bought on a residents salary. I wrote them as well.
I write everyone. If it is a program I am not very interested in I may only send a letter to the PD. But if I am half way interested, I sent the thank yous to everyone involved in the interview and send them right away.