Well, I think it is really left to your own discretion when you get into the interview and get a feel for the flow of your interview and the interviewer's personality...Of course, I would stay away from "risky" jokes (jokes dealing with ethics, religion, etc.). I can give you an example of something that happened to me that kind of made me more aware of the whole "joking dilemma." I was calling to schedule my interview and this is what happened:
Admissions office: The earliest day we have open is October 31, would that be ok with you?
Me: Yeah, that's great. Oh, that is Halloween. Should I come dressed up as something?
Admissions office: Dead silence.
Me: I was just kidding.
Admissions office: Yeah, that's fine. Anyway.....
I know that I probably wasn't talking to the Dean of Admissions, but after it happened I was definitely more careful about when I would use jokes..