NO! NO! NO! We went in November during "Jersey Week," which will be more crowded that when your going. I regretted it. It rained and this was my experience.
(1) You have to pay for the dinner when you book WHETHER FANTASMIC IS CANCELLED OR NOT! (THat means if it rains, and it rains like the dickens in September, you are stuck with the dinner and no show.)
People seemed to linger over their Hollywood and Vine dinner when it rained, so we had to wait 20 minutes AFTER our PS time (after arriving 10 minutes before the appointed time) in the rain for our table to be ready. We couldn't really do anything else, because we didn't know how long we'd be waiting.
(2) After we ate (and we lingered because we were soaked and cranky), there wasn't enough time to catch any shows before Fantasmic. Many were done for the day.
My beef with MGM in the rain is that there is very few yound-kid friendly rides or attractions to duck into when it rains. The Honey I Shrunk the Kids Playground shuts down and most of the rides are too "thrilling" for my kids. I hate shopping. So, we ended up waiting anyway, just to see if Fantasmic was happening or not, so we could decide whether to just leave that park or not.
(3) The reserved seating is off to the right side of the stage, so are not the best view anyway. Crowds are light enough at that time of the year that the standby times shouldn't be an issue and you're likely to have a better seat.
If your heart is set on the reserved seating, my advise it to check the weather channel and book the package the day before or the day of. (I would have preferred to be at either MK or Epcot than at MGM in the rain.)