Ditto- don't ask a question just to ask a question. I haven't ever asked any. I don't think it's necessary to plan out questions. People ask that more as a courtesy and as a way to end the interview more smoothly in my opinion. If a question legitimately sparks into your head during the interview due to the particular questions asked and conversation had, then sure, I'd ask it. Otherwise, I've just always responded with a polite no, that the information sessions and students have been extremely helpful and informative.