Well, first I should figure out your definition of a "seat". If you mean an actual seat, you probably going to be disappointed. Even if you actually got a bench, someone will probably stand in front of you and ruin your view. Your best bet for a bench seat with a view is at Epcot. Now this depends on the time of year that you are going but we can typically get a bench seat in World Showcase 1/2 hr to 45 minutes before Illuminations. We try to get one as close to the railings as possible so people won't stand in front of us. However at the other parks, if you get a bench seat, people will stand in front of you. My suggestion is to grab a "seat" on the sidewalk edge along the parade route and wait there. It's considered bad parade manners to "hold" spots so don't expect to be able to sit there and hold spots for the rest of your party while they are of riding rides...people will complain.
We only watch the parades/fireworks at MK and the fireworks at Epcot so we tend to try to get as close to the front of the park as possible to watch. That way, when the fireworks have ended and there are massive bottlenecks at the gates and lines at the buses, we have a headstart! We also always watch the finale of WISHES in the bus line!!!