I didn't either. Most of the interviews I was there for several days so I went the day after or the day before I left and thanked the main people involved in the process and thanked them in person and mentioned something I will take back with me about the school. I think I got a very positive response. Mark even told me when I went and thanked him: "See you next year, [DesiDentist]."
I talked to Su-Ann Zarrett at NOVA. UPENN i went to the admissions office and thanked all of the assistants (Elsie and Ramona) for putting up with me (I called UPENN the most during application since my file was misplaced) and Corky.
So I felt I didn't have to send thank you cards. But it is nice, but not necessary. The goal is to leave a positive impression, I believe there is no way to leave a better impression than in person.
Good luck,
DesiDentist