I am with Simul on this one. A bit old fashioned admittedly, but I do think there is something that you portray about yourself through a nice hand-written card to thank someone. Even to a program you have waning interest in after interview day - I just tend to think it is a nice thing to do to take the time to personally thank the interviewer. There are things we do to increase our odds of matching, and then there are things we do just because it is the right thing to do. I think thank you cards probably fall in the second category. Not that you are doing something "wrong" if you choose not to write them...it's kind of like volunteering to take call during Christmas because you know that your fellow residents have small children at home and you do not and are not planning to leave town. You don't have to, but it's sure nice and a good reflection on you and people remember stuff like that.
Like Simul, I wrote each interviewer a note. But I also emailed the PD right after the interview to thank them, b/c as stated above, you are more likely to get a response by email and dialogue is a good thing. I emailed my top 3 again sometime in late January and emailed my top choice once again right before the deadline to let them know I was definitely ranking them #1.