If you use FastPass, you will rarely wait more than 10-15 minutes for anything. Whether there are short lines or no lines will vary by day of the week, time of day and other cyclical issues. May does have higher crowds than September / October, but not by too much.
In particular, I'd recommend spending time at EPCOT earlier in your trip. The closer you get to Memorial Day, the larger the crowds will be. Between the Flower and Garden festival going on at Epcot, and many schools taking field trips, we found that park to have some of the larger crowds we saw during our stay last May. By far, the longest standby line we saw during our May trip was 65 minutes for Test Track. The Mountains at MK were usually 10-20 minutes, while ToT and RnR were 10-15.
Also, keep your eyes open for info on Star Wars Weekends at MGM. The schedule should be annouced in Feb or May, but the time you are visiting normally encompasses the event. Typically, during the latter part of May and into June MGM will fill the weekends with special Star Wars fan events. The "weekends" run from Friday to Sunday, and will definitely cause crowds to peak when held. If SWW sounds like your thing, make sure to include it in your touring plans. If not, avoid MGM on Fri - Sun.