The first time you arrive at a KOA, they'll ask if you have their membership and will explain the discounts. We only join in the years we know we'll be using them a lot. We had two years in a row where we visited family and went to reunions where the nearest campgrounds were KOAs. We had some decent savings those two years and they had some kind of discount the third year that we also were able to use. Don't feel you have to join or renew.
None of the memberships, whether KOA or PA, are good if their campgrounds aren't where you want to go! As long as you have a good campground directory you don't have to restrict yourself to membership camping unless that's what you want to do. We use our Good Sam or FMCA discounts and they serve us pretty well. Unless you want to travel from one membership campground to another, I wouldn't worry about any membership until I get to a destination where one is usable.
ArdraF