We did the package thing in September of last year. About 15 minutes before the show started, they opened the "reserved" area up to everyone, so we ended up sitting with people who just walked in at the last minute and sat down next to us.
Depending on the time of year, reserved seating may not be necessary. On the other hand, the year before we were there in November (Non-Thanksgiving week) and people were lined up almost to the front entrance of the park an hour before the show. I couldn't understand what the line was for, so I asked and was more than a little surprised to find out it was for Fantasmic. So, we skipped it.
And, sort of off topic, we ate at Hollywood and Vine last year. I thought it was just so-so. My wife hated it. But that's just our opinion. I'm sure others like the food and the atmosphere.