Even though September is a slow time of year, I'd still recommend using FP+. Why not?
Even though most of the lines can be fairly short in September compared to the rest of the year, there can still be significant wait times for the most popular attractions (like 7DMT). You'll see some posts that will say "On my September trip, all rides were walk-ons" but that's an exaggeration. Maybe first thing in the morning, but as the crowds build during the day, you'll have some waits.
If you don't want to tie yourself down to FP times, remember you can always change them, or you can skip them if you want. But it sure is nice to have them!