there are so many different websites that comment about this and they differ in what they say, but for the most part, they encourage you to write thank you cards to *everyone*--all interviewers and the program coordinator, plus the chairman and PD, if they weren't one of your interviewers.
i think the necessity of writing the cards can depend on a lot of different things: how much you want to go to the program, how competitive of a candidate you are, how personalized you can make the card, etc. but i think a good general rule is to send them to at least your interviewers, which will often will include the chairman and PD (though at my first interview the chairman's "interview" was a Q&A mostly and he explicitly told us not to write him a thank you card...and i wasn't sure how exactly to take that--so many ambiguities...)
