Hi Jaynee!
The only downside to visiting Walt Disney World in the summer, if there is ever a downside, is the heat and humidity. Surprisingly, the heat and humidity doesn't typically differ all that much from June to August, although June usually has much more precipitation than August. I think making you decision based on crowd attendance is probably the best way to go.
Your best bet for lower crowd attendance would be the first week of June or the last week of August, although my vote is for the last week in August. Events such as Star Wars weekend make June a great time to visit, but will undoubtedly make it more crowded. The last week of August will see much smaller crowds due to the fact that most schools will be back in session throughout the country. Another great benefit to traveling that last week of August, it's cheaper!
Room prices are always lower during periods when crowds are predicted to be lower and that is always a good indication on times of the year to visit for smaller crowds. For instance, in 2013 a standard room at Port Orleans Riverside in June will cost $196.00 a night. The same room in August will cost $170.00 a night giving you a savings of $156.00 for a six night stay. Add that with one of
Disney's Special Offers and you could have some substantial savings on your hands, keeping more of your Disney dollars for Dole Whips!
I hope this helps with your decision and please let me know if you have anymore questions while planning your next Walt Disney World vacation!
Happy Planning!
Mandy