I agree with the PPs that said it really depends on when in May you are thinking of going.
On or After May 22: May 22nd (the 24-hour day) is the kick-off to the summer season, when park hours are longer, entertainment schedules are fuller, and fewer refurbishments, but the crowds are higher. The weekdays might not be to bad, but with all of the new 60th anniversary entertainment, I would expect moderate-to-high crowds starting on May 22nd.
Before May 22: Prior to that, it won't be that bad in the parks, especially on weekdays. There are some days when the schedule is little off (e.g., DL open slightly earlier, but closes at 7:30 pm). It's also worth noting that there are a number of refurbishments happening right now (e.g., Peter Pan, Matterhorn, Soarin') due to the upcoming 60th, so if you go in early May, those attractions may still be closed. Also, fireworks and F! are only presented on weekend (Fri to Sun).
May 7 to 10: Lastly, it's worth noting that May 7 to 10 is when the Tinkerbell Half Marathon is scheduled, so expect high crowds during those days.