Everyone is different, so my comment here is not a criticism, just a different perspective. We are going to the MVMCP (first time) during our trip in December, and I'll be having myself & my family grab something early & quick for dinner rather than taking the time out of the special event for a sit-down dinner. In my opinion, the event only lasts a certain amount of time, and I'd like to make the most of it, rather than eating. I know that MNSSHP is different than MVMCP, but some of the things I've read about MVMCP state that if you want to meet some of the characters, getting in line for them by 530pm is not unreasonable,and I think that MNSSHP actually offers more to do than the Christmas Party. So with that in mind, knowing that I might not get to do everything I want to do (parades, taking photos of the decor, character meet & greets, special holiday shows, a couple of rides, special treats, fireworks, shopping event merchandise, etc), I feel like it makes the most sense for me and my family not to have reservations for dinner on a Party night.
**Again, completely understand what works for me might not work for you, just offering a different perspective that maybe you didn't think about. If you do want a dinner reservation, then I'd go for as early as you could get one.