I went last October from the 15-17. We stayed on site so we used FOTL so we never had much of a wait. I noticed the closer it got towards the evening hours and Halloween Horror Nights the more crowded it got. I saw waits on some of the rides as long as 60 minutes.
The park won't be crowded until around 5:00 PM when the HHN guests start arriving for Stay and Scream. If you are going on a weekday when their isn't HHN, then it won't be crowded at all during the day.
During the day, the parks will not be that crowded. As phamton said, closer to 5pm the crowds will start to get thicker. I have found that I didn't need it during the day.