Both times we saw it this week it started at town square and went towards IASW. On a busy Friday we walked in the park 10 minutes before it started and got a great second row spot on the curb in front of the train station. Further up Main Street and beyond was pretty packed.
Also FYI, the first MSELP parade starts at IASW but the second parade of the night goes the opposite way. We found it was much easier to get a good seat for the second parade.