I agree with AimeeM. Our last two trips were in the off-season, and there were seats available (albeit not right in the middle) when the show began.
If you want prime seats, you'll have to arrive 45 minutes early. No way to avoid that. The dinner package seats are the section to the far right, and the last section to fill up with walk-ins is on the far left.
The major complaint that you will see regarding the dinner package is the seating. The dinner package guarantees you a seat--but certainly not a prime seat. If you are already planning dinner at one of the package restaurants (Brown Derby, Hollywood & Vine or Mama Melrose's), you might as well get the package. I don't believe you end up "losing" financially on the package. But for the time of year you are visiting, the package is likely to be irrelevant with comparable seats available at showtime.
If you're going to be in MGM on the day you plan to see Fantasmic, use the crowd levels to predict how busy Fantasmic might be. If standby times for the prime attractions hover in the 15-20 minute range, you should be able to walk in just before the show starts. But if the park seems much busier (say 45 minute standby times), plan to arrive 30-45 minutes before showtime.