For MK a lot of people are sitting on the curb for Spectro about an hour or more before hand and then they just stay there for the fireworks in a lot of cases. The afternoon parade in MK is pretty much the same. I have found that if you try to find a spot along the route, closer to the ending area you have better luck.
IllumiNations in Epcot brings people out waaaay ahead of time. If you want a spot right up front you have to stake out a spot really early..over an hour ahead.
MGM isn't so bad, at least in my opinion. Maybe 30-45 minutes beforehand, depending on where you want to be.
And AK it wasn't bad at all. We finally did the parade for the first time in late Nov and found a really good spot about 20 minutes before it started.