We've watched them from toontown and from behind the castle and on the bridge in tomorrowland.
The show is made to be viewed on Main Street. If you view it anywhere else the effects will not be the same. For example some of the fireworks when viewed on MS combine to make a single image, if viewed anywhere else the fireworks don't combine properly and just look random. On MS the fireworks are centered around the castle, the castle is a very important piece of the show and if the fireworks are viewed elsewhere you're missing a huge part of the show IMO.
Yes if you view it behind the castle you'll see that some of the fireworks are fired from the fantasyland buildings, and the rest from in back of toontown. It is interesting, but IMHO the show is best when viewed on MainStreet where it is made to be viewed.