We've done this on 4 trips, and will make it 5 next week. We generally try to get a 7:30 PS, but have had 7:20 and it works fine. They have a rule that you can only keep your table for 2 hours, I'm guessing since 6PM PS's were trying to hang on until Illuminations.
Twice we've had a patio table, which is perfect. Once we were inside, once we had a table along the side. In those cases, R&C has a special viewing area that your server will take you to minutes before Illuminations starts. It's not as nice as watching from your table, but it beats hanging on to a crowded spot for a long time elsewhere on the lagoon.