I think it more depends on what day the party is on. Friday and Saturday parties are generally more crowded than Sunday parties. Of course, I went to a Friday party where crowds were supposedly high, but I didn’t have a problem with it. It also depends on what you want to do at the party as to whether or not you have to wait in line. If you want to meet all of the characters, then that’s one of the most crowd-drawing, time-consuming things to do. If you just want to collect candy, ride attractions, see the parade and fireworks, and get photos with a few favorite characters, it shouldn’t be bad at all.
We strategized and got pictures with Mickey and Minnie immediately after the second Boo to You! Parade and there was no wait at all (I’ve heard that if you try to get your photo with them immediately, then there is usually a long wait). The seven dwarves are the most popular, and we decided to skip out on that.