For parades, anywhere along the parade route is good. If you plan to make a quick exit after the Electrical Parade at DCA, get a spot in the Sunshine Plaza area. It's at the front of the park and the parade goes by there first. It's also the first place to fill up.
As for the nightime shows, I'm not sure where the prime viewing will be for the 50th fireworks, though somewhere in front of the castle would probably be the place. I like watching from the Rivers of America area (where Fantasmic is) as it gives a different view and if Fantasmic is playing, I never miss it so I'm over there anyway and I'd never make it to Main Street in the few minutes between the first Fantasmic ending and the beginning of the fireworks.
For Fantasmic, unless you're doing the dessert buffet on the balcony, front row center can't be beat. But these spots are usually taken hours before the show.