Generally, the last spot to fill is at the very beginning of the parade route. More specifically, the area right in front of Harmony Barbershop. Periodically, the CM roping the route will improperly rope this area reducing it's size. But, generally it is a large area that can accomodate several folks and typically stays empty until very close to the beginning of the parade.
The down side to this is that some of the Magic is lost because you see the floats in the prep area each time the doors open. But, that doesn't dissuade us. We enjoy being able to sit down 15 minutes before the parade and have a great spot.
Also, the characters on foot will come right to you since you are the first guests they see.
But, if this is your first time seeing Spectro, I'd suggest a spot further along the parade route. If you don't mind finding a spot early, Main Street really is the most magical place to watch any parade.