We have never done this at WDW, as we try and make our reservations at the off times and still expect a small wait. However, on our last trip to DL we showed up about 30 min early for our reservation and asked if we could be accommodated. They said they would seat us as soon as possible. Sounded good to me until several walk ups (at least 8 families, some with the same size as ours of 3) were sat before us with no reservation. I was prepared to wait, as we did show up early, but then when we had waited another 30 min past our original time ( so an hour of waiting at this point) I finally talked to a manager and explained what had happened. He told me they would seat us next but that is would still be another 20 min. I politely declined and asked him to remove the cancellation fee. He did take care of it for us, but I would not make reservations expecting this to happen. We only book the restaurants that really matter to us, book in the off times of the day, and also make sure we don't have any fastpasses that would conflict if we would be late due to having to wait.