Well, front and center is the best viewing place, of course! And it's not terribly difficult to arrange this with a little planning.
All you really need is a blanket and a commitment from one person in your group to be at your area at all times. I often like to take something to read with me, and get there by 5 p.m. for the 9 p.m. show. Yes, 4 hours in advance. Put the blanket down and read.
Then we take turns taking the kids around to various rides until 9, and also get our dinner "to go" and eat it at the edge of the river.
If you can bring anything to support your back, then do so.
Anyway, that's really the easiest and cheapest way to get the best spot.
Another *giant* tip. When Fantasmic is over, do not leave your spot to try and make it over to see the fireworks from Main Street. Everyone tries this, makes a huge traffic jam, and then you'll be stuck in a horrible area for viewing the fireworks. From your spot at Fantasmic, you can see the fireworks just fine!
s.