You can see the fireworks from behind but you'll be missing the "story" part of the show.
I can't stand around for an hour so we've done 30 minutes and had a good spot. We've also walked into MK during the show (love park hopping) and watched from the train station. If you can manage it, don't miss Happily Ever After. It's a really cool show. You don't want to be too close to the castle but in the hub area if you can manage it. The train station was too far to really see the projections.
Are you traveling with kids? If so, my recommendation is to position yourself behind some type of railing or taped off area. Then you won't have some tall person squeeze in front of you, blocking the view. Be aware that if you are not behind the tape barriers, cast members will shoo you out of the area. Make sure you find a spot where at least your feet are inside the tape.
Once Upon a Time is also a nice show but less crowded. I really enjoy the projections - they are very well done.