As far as cancelling interviews...I think everyone does this. You should reply to invitations quickly as the interview offers come in b/c you might miss out on your desired date if you don't. Then if you later get more offers you'd rather attend at that time, cancel as soon as you know.
As long as you cancel EARLY, as soon as you know, you will leave that date available for them to book someone else. It is very poor form to just not show up for an interview without canceling - people did this to some of the January interviews I attended and I think it is rude and you never know who knows who amongst these programs - you'd hate for the chairman of your #1 to bump you down the rank list b/c chairman of your no-show program happens to be his best friend and you made yourself look bad by not showing up for a scheduled interveiw without canceling. On the other hand, I think cancelling in a timely manner is OK. No you don't have to give them a reason unless you want to.
in response to jc - you only really need 2-3 interviews before you feel comfortable with it, so after 2-3 of your lower-teir, you can start booking your top choices. If you leave the top choices till last you may be kinda bunrt-out. Most people reach that burnout point around the 10th interview it seems, so try to peak in the middle of the interveiw season with the top choices in the middle. I scheduled the ones I wasnt' too interested in, in late January, so I had plenty of time to cancel if I decided I booked enough places I liked by mid-December. This is when the interveiw invites slow down and you should have a firm handle on your schedule by mid-december.
] start)