No and you don't have to cancel. It's nice because it allows them to reallocate numbers for the day but since you are not booked on a specific bus or a specific time when leaving MCO it isn't critical. They won't wait on you, since they have no idea what bus you'll get on anyway.
Now, if you registered for pick up you may want to let them know you are not there. They do schedule folks on those buses. What they'd do with someone who never checked in is unknown. Seeing how Mears runs ME and has a different system that links to Disney I wouldn't assume much of anything. I would hate to think they are looking for you at departure to MCO.
To report on what happened in my situation re: cancelling and DME, when I called to cancel my return trip to the airport (out of courtesy), the CM said that someone had already called to cancel it. I asked her for future reference if that always happened and she said no. She said it's off and on.