No, they wouldn't be waiting for you, but they do base the number of buses going to particular resorts based on the people who have reserved a ride at a particular time. So...while not 'mandatory' to cancel your reservations, it is nice to let them know. Heck, I had DME reserved but found that my ds was going to pick me up. I called them and told them I didn't need the ride to WDW but still needed the ride back to the airport...they really seem to appreciate the call.