for the last couple of years, they have recommended to get to the theater 60 minutes before show time, even with the tickets. We generally get there 45 minutes ahead of time, and have our choice of seats in the reserved section (we like to sit near the top, for a fast "getaway" once the show is over.) I would not advise waiting until 30 minutes to get a seat since sometime between 30 & 20 minutes before the show they drop the ropes on the reserved section and let people from the standing-room-only section fill in the remaining empty seats in the reserved area. I have read several reports from people who had reserved seat tickets showing up at 30 minutes and having no seats left.
One thing about going during really slow periods...that usually means the Fantasmic show will be even more crowded because during the slow times they cut back to only one or two shows in the entire week. So though the parks might be nearly empty, everyone who is there that week will ALL have to try to get in that same show. If you really want to see Fantasmic, I would go for the dinner package tickets.
Agree Mama Melrose is very good -- slow service, but good food.