I don't know about other school, but at OKSU the interviewers sit and read your file beforehand... then they interview you... then they sit for a few minutes after and they decide your score. After that, your file leaves their hands and they have nothing else to do with it. So you would be safe to send a card at any time without the possibility affecting your score in any way.
I still haven't decided yes/no either... one person said it would be buttkissy, and another said she thought it was a nice gesture (both committee members, past or current).