For crowds, the very start of summer season 2010 worked very well for us, so I second the "before school breaks" idea. We were in the parks around and on June 11--the "public premiere" of World of Color--and I expected the parks to be incredibly busy, what with so many people online saying they HAD to be there for the first show. The parks weren't empty by any means, and we did have to leave early one night because of a grad nite (which was OK because we had time to rest up for the next day!). But the crowds weren't bad at all, and the full array of summer entertainment officially started that day, even though it wasn't peak-season busy. So the second weekend in June was almost perfect for us.
(It was COMPLETELY different when we went around 10 calendar days later in June 2009. The lower tier APs were about to be blocked out for the summer, kids were out of school, and it was much, much more crowded. But you've already ruled out later June so you're probably fine.)
I've heard about massive crowds in general in October, with all the special Halloweentime offerings and so many events in Anaheim. Currently, however, World of Color is running daily, even though it's off-season. Plus, nowadays the nighttime shows pretty much run all weekends (Friday - Sunday), provided maintenance, weather, or early closures don't prevent them. And Halloweentime is really high season as far as crowd attendance. This year they pretty much HAVE to run World of Color in DCA just because they're closing
Disneyland early on Tuesdays and Fridays in October, for Halloween parties. Anyway, we can't KNOW what will happen next year, especially since WoC seems to have some technical issues already, but if WoC is the primary reason you don't want to go in October, I think you're about as likely to see it in October as you are in June.
Still, for crowds, I think early June will probably be lighter.
Have fun planning!
