Go more then once for every park. Get it from a few vantage points. Absolutely, go early for a good seat. For the best spots you need to wait 2-3 hours at least for MK if they have Spectromagic on the same night. It's worth every minute. Some use the time to eat, snack or just wind down from the long days' activities. I find it best to take an afternoon nap especially with young ones; so, noone is crabby waiting so long. Keep everyone occupied. You have to be a little creative with the long waits.
My personal favorites are Italy for Illuminations. Some countries block your view of the globe. For Fantasy in the Sky, I like the cement benches in front of the castle. This is the closest I would be to the castle.
By all means if you can afford it take both cruises for the MK and Epcot fireworks. We are into sharing now and it keeps cost very low.