I have been to WDW in both the middle of May and first part of June. Middle of May trip it rained every afternoon. We just planned on parks in the morning, room in the afternoon and back to parks later. Beginning of June was almost identical although we had one or two days without rain.
That all being said.....during times of rain can be one of the best times in the park if you don't mind getting a little wet. Lots of people head to their hotels and the crowds can be much less! Just bring the ponchos with you and you'll be set.
I've can't recall of a single time at WDW where it actually rained all day - just mid to late afternoon for a short time. I can't say the same for Disneyland though.
Some years you can get no rain then because of drought. Sometimes an all day rain. Normal/average pattern is you can get 2 to 4 thunderstorms daily mostly in the afternoon and evening that usually don't last too long (30 minutes each). That is actually a pattern Florida has from May to October.