We've been taking DS since 2002, and we've always gone in May. The first time we went was the first week of May, and it was perfect - low crowds and gorgeous weather. I wish I had pictures to share, but I didn't have a digital camera at the time.
After DS started school, we started going the last week of May (the week before Memorial Day) so he wouldn't miss any school (well, technically he misses the last week, but because it's just a bunch of half days and the grades are already in, I don't really count it

).
Every year the crowds get heavier. The last time we went was in 2007, and we were amazed at the crowds. I know that Spring Break travellers would scoff at my interpretation of "crowded" - but to us, with nothing to compare the crowds to except late May crowds from previous years, it was packed. I don't remember what the UG predicted the levels would be - I *think* it was mostly 6's and 7's, with 8's on the Friday and Saturday before Memorial Day (our departure day). We also made sure to go to the Least Crowded Park each day (according to TGM).
I went back and looked through my gazillion trip pictures and found a couple that show the crowds.
Even though it was more crowded than usual, it really was very manageable, and we had a great time. And once you got out of the "main" areas of the parks, the crowds really thinned out and it wasn't bad. But if these were 6 and 7 level crowds, I'm pretty nervous about 9's and 10's.
This was in AK, two Sundays before Mem Day, at 11:30.
Magic Kingdom at rope drop, the Wednesday before Memorial Day.
MK, 1:30
.