I'm with North of Mouse; for us, a dessert party isn't worth it. We don't eat large quantities of desserts, so if I wanted I'd buy something special, add in some nice drink, and stake a spot.
September isn't really a super busy time. I'm going the 10th-24th (short trip), and touring plans has the crowd calendar between 1-4 most parks most days, except Epcot on the Saturday as a 5. Knowing that, I'm sure it's quite easy to find a nice spot to watch fireworks.