I can't speak for Epcot Forever, since it's a new show. We've done 4th of July about 6 or 7 times now, it's our favorite. If Epcot Forever is similar to Illusions with respect to crowds, then it's no big deal whatsoever. A few years ago, we stepped of FEA and got a decent viewing spot at 8:55pm, with the show starting at 9pm. Were we right on the railing? No. We were actually on one of the bridges heading back toward FutureWorld, which worked perfect. We still had a great view of the show and were the first ones out when it was over. Since the World Showcase is so large, there's plenty of viewing. And the nature of the show (at least Illuminations) doesn't require a "front row" seat.
As for MK, well it is what it is. The hub is only so big. If you want a good spot, be 90-120 minutes early. A so-so spot on Main St you'll need to be 60+ minutes early. If you don't care about that, Poly beach is a fabulous place to watch. Not crowded at all, you can walk up as they start and see. Pretty cool view too.