I think it also depends a lot on when you're going and where you want to watch. Seated spots (aka front row) tend to go first, but areas in the sun will last longer than areas in the shade. Also, the ends of the parade route (so either the very beginning or the very end) seem to fill up last. I was able to find a spot on the curb, right near the start of the parade on Main Street (across from the Opera House), in the full sun about 20 minutes before the start in April when the parks weren't that crowded. During busier days, I would guess you'll need to start looking much earlier (~1 hour) if you want a front row spot in the shade.