If you arrive earlier you may have a better chance for a window seat. It's my favorite restaurant at WDW, and one of my top of all time

somehow we've always been there on a celebration of some sort (birthdays, anniversarys) so they had that in the system and had already prepared the window seat (w/ confetti and all!!)

so, if you're celebrating something it wouldn't hurt to have that on the reservation. Even without a window seat you'll be able to get up and enjoy the fireworks, it'll be beautiful. Have fun!!