LOL Honestly I called up 2 weeks in advance and got the prime seating. I booked Ariels Grotto, Blue Bayou, and Goofy's Kitchen. Id still call in adavance as its definately not a bad idea.
For Fantasmic though, it is 30 days EXACTLY to the show date that you need to call, and first thing in the morning when the office opens, and then you fight with many others to get prime seating. But as for dining, no you should be alright.