I was staying at BLT on a solo trip and interested in eating at Grand Floridian Cafe. I found a reservation available in the app for about an hour and a half later. That was longer than I had hoped to wait. It appeared that I could join the waitlist as well, but I wasn't in range for that. So, I walked over to the monorail and took a ride over to the Grand Floridan. By the time I got there, the reservation was gone, and so was the option to join the waitlist. Disappointed, I walked up to the desk and asked if I could get a table. They had to write my info down and said they'd try to accommodate me. In the mean time, I walked upstairs and grabbed a drink at the Enchanted Rose. About half an hour later, I did get a text that a table was available. I finished my drink and headed down to enjoy that lobster thermidor burger.
My advice is to look for a reservation. If one is available, you should probably grab it. If it's later than you like, go ahead and take it and then show up at the desk and ask if they can seat you early. I doubt you'll have the opportunity to join a waitlist if a later reservation isn't available.