From our trip a couple of days ago: if they have two MSEP showings, and if you can stay up late, it seems you can go to the second one pretty much anywhere in Frontierland or Liberty Square without waiting at all. We were right by the stockades in Liberty Square (something about that sounds ironic...) and had a great view of the parade coming head-on then turning (it ran from Main Street to Frontierland - I think the first one ran the opposite). Although we were there a few minutes beforehand, other people showed up by us just before the parade (I mean JUST before) and there was plenty of space, and lots of empty space around. In fact, it almost felt weird to have so few people around - I think sometimes a bit more of a crowd gives some energy... Still, it was nice to watch a parade without having to worry about staking out a spot ahead of time, and we had an absolutely perfect view. People were staking out spots on Main Street from just after the fireworks, so it was strange to see all those people waiting 45 minutes when there was no wait at all just down the parade route a bit.
In the past, we've usually gotten a spot well ahead of time. We have pretty much always been in the Liberty Square area.
If you can position yourself in the right position near a bend in the route, you can see floats both head-on (as they come toward you) and then to the side as they pass.
The parade takes a while to travel its route, so if it's a 3:00 parade, it might be just starting to get to the far end at 3:15 or 3:20 or so (that's an estimate - someone probably knows for sure). Consider that when deciding how long you're willing to wait - getting one near the head-end of the parade could save you a significant chunk of time. Also consider that if you are planning to leave or be somewhere after the parade - if you are near the end, you could be running several minutes late. Also, realize that right after a crowded parade, lots of people are dispersing (and possibly going the same way as you), so if time is an issue, positioning yourself along the route closest to where you need to be next could help.