That is a good question, and I supposed depends quite a bit on just how popular the new parade is. If you absolutely HAVE to watch from Main Street, I would say at least be IN the area an hour before the parade, and stalk the curb line. When you start seeing other people lining up, that is your cue to get a spot.
However, if his parade follows the entire route of the afternoon parade, and will be going through the Frontier Land area of the park, I would say go for a spot back there. We have gotten good, and sometimes even perfect spots, even for the night parade, in Frontier Land as little as 10 to 15 minutes before the parade starts.