I've had to cancel or reschedule maybe a handful of shops in the years I've been doing this, and have only gotten a hard time from two schedulers. On one, I had no transportation due to my car breaking down. I offered to do the shop a few days later, but she declined. The shop was still sitting the following week, and they had to offer a bigger bonus for it--when I'd have been doing it for the original fee I'd agreed to. That hasn't affected my getting shops from them, though. The other time was when I asked to reschedule due to hazardous weather conditions, and the scheduler was a real SOB about it. I did the re-scheduled shop, but I don't think I've ever done another one for that MSC.
The other times I've cancelled or rescheduled were when my sons were hospitalized (at different times). All the schedulers were very understanding, and my ability to self-assign or be assigned shops hasn't been affected.
Anyway, a long-winded way to say that if you prove yourself dependable over the long run, and you have a
very good reason for cancelling or rescheduling, I think most schedulers are OK about it. And remember that this business is a two-way street. A good scheduler knows that!
I learn something new every day, but not everyday!
I've learned to never trust spell-check or my phone's auto-fill feature.