But you do have to get there no fewer than 45 minutes before showtime or you give up your guarantee of a seat.
Depending on the day, sometimes people line up for 2 hours or more. The voucher you receive at the end of your meal will state that you must be at the special entrance no later than 45 minutes before showtime. Around 30 minutes before showtime, cast members will remove the rope separating the reserved section from the general seating, and at that time ANYONE is permitted to sit in that section.
When I last saw Fantasmic, it was during a relatively dead time of year, but people started lining up before 5 p.m. for the 7 p.m. show. I arrived right around 6:15 p.m. (voucher in hand) and very easily got seats in the reserved section. I was told that at 6 p.m. (1 hour before showtime) there was an announcement that it was standing-room-only and that people should stop milling around looking for seats. When the CMs removed the yellow rope at 6:30 p.m., a huge sea of people flooded towards us and in minutes every single piece of bleacher bench was covered with people. Anyone who had the Fantasmic Dinner Package but arrived after that point was out of luck and without a seat.