My T has a 24 hour cancellation policy. They even keep track at the reception desk of the exact time that you called so that they know whether or not to charge for the missed session. If I was unwell, I'd imagine how anxious and upset I would feel about having to cancel my session - especially if I found that I felt better by the time the session would have happened. I like the idea of asking to reschedule the appt for later in the week, if that's a possibility.