I think it is better to do it ahead of time. Recently I had to reschedule a shop because I screwed up on the time frame. They did reschedule but warned me that it would result in a citation next time. They sent an email stating that cancelling or rescheduling before the shop was acceptable.
I agree with Tucketgirl. If you act in a responsible manner, offer alternative dates or at least not try to hold on to it, then most MSC are accommodating. If you hold on to it until the next day, find that you cannot perform it, then cancel without alternatives then that's a day wasted that someone else could have picked up the job and performed it.
Do not read so much, look about you and think of what you see there.
Richard Feynman-- letter to Ashok Arora, 4 January 1967, published in Perfectly Reasonable Deviations from the Beaten Track (2005) p. 230
