It's hard to say when the best time to go is. IF World of Color premieres 6/11 then probably the whole rest of June will be very, very busy. (But even if it premieres then, there's a great chance that there will be some preview days before then.) Conversely, if you want to see World of Color, waiting until 6/11 and later may be best.
Blockout dates would say to go June 16-18 or 23-25, but I think before 6/11 might be best this year.
Disney expects June 21-22 to be crazy-busy, and I think the last week of June will be crazy too.
I don't know. There are so many weird factors involved this year--normally earlier in June is better, but with World of Color I'm not sure there will be THAT much difference. Unfortunately we probably won't have any good ideas of how things will be until mid-May or later, and that's rather late to be planning a vacation.
So I think you may need to just pick a date, make sure you get to the parks before opening each day, and come up with plans of attack for whichever days you pick as your vacation draws closer.
Have a great trip!