I could be wrong, but I think the fireworks are launched from behind NFL. so, although the view would be mostly unobstructed, you won't have the full effect of the framing of Cinderella castle.
Wishes from above The Beast's castle, is a good place to watch. We have done it several times. There are fireworks going on all around you, you not sure where to look! We can usually find a place to sit as there are hardly any people back there, even right before they start. Then when they are over, and everyone is trying to get off of Main St, Fantasyland is wide open and you usually walk right on the rides.