I wanted to jump in and offer a few hints from the perspective of a coordinator.
1. People cancel or reschedule all the time. We expect it. We all know how tough it is to coordinate schedules with other programs, and no program coordinator or director is under the impression that you're only interviewing at their institution.
2. If you need to cancel for whatever reason, do so as soon as you know. We often get more cancellations later in the season as applicants run out of money/steam and realize they have enough programs they want for their rank list. But give us a chance to offer your spot to someone else.
3. If you are cancelling for good, you can go ahead and check the withdraw box on ERAS. That's helpful to us. BUT...do not simply withdraw and then not call or email to cancel. It's rude. And remember - bad ettiquette can come back to haunt you.
4. If you need to reschedule, just email or call. But understand that interview slots fill up quickly and it might be hard to find the date you want. Our program will "waitlist" a reschedule if we can't accomodate and will let you know if anyone cancels from the date you are trying to get. We're also pretty flexible about adding spots if you absolutely can't do any other day (especially if you're coming from far away), but not all programs have that flexibility.
5. I second what someone said about not asking for interview dates until you're been offered an interview. I tend to find those emails or calls pretty presumptuous and it does not put you on good footing with me. To us it sounds like "I know that you'll be offering me an interview because I'm just that good, so I'd like to start working you into my schedule now".
6. Finally - RELAX! Even the things I mention above that you shouldn't do...it happens. We are not out to get you. We really do want everyone we interview to do well. As long as you don't treat us like garbage, we'll pretty much give you the benefit of the doubt! From my personal perspective, I'm as invested in you having a good experience interviewing at my program, as you are in having a good interview.
Hope that helps!