Call the day before you leave home but....
Also call the day before you leave Disney. We changed resorts, and when the town car was 30-40 minutes late I called TTC. The driver was sitting at CBR, but we were at CSR. A mad rush to the airport. This happened before 9/11. If it was after, we would have had trouble making it in time.
BYW, Tiffany made good on their mistake, and the bad attutude of the driver, who had sat waiting at the other resort. We will be using them again in October.