I, myself, avoid WDW June thru September like the plague. I understand that some people don't have a choice as far as timing goes, but, for the love of everything, don't flush thousands of dollars down the toilet to sweat your butt off while dealing with an annoying amount of people. A good touring plan can manage the crowds but it becomes increasingly unbearable when it is 90 to 100 degrees with 100% humidity. This kind of heat mixed with hordes of sweating people can take its toll on even the most intense WDW commando on earth.
I can't tell you how many people that I've talked to say, "we went one Summer and would never repeat that painful experience again." It's disheartening when someone's impression of Disney World is marred by such a situation. Choose your battles wisely people.
If you have to go during the Summer, yes, get there at opening and get out by 11 AM for a break. Return in the evening for the less popular attractions and, mostly, for the evening entertainment. PACE YOURSELF. You simply cannot take your time schedule for granted as you would have the freedom to do in the off-season. Be diligent with your plan and follow through with it.
I've been to WDW more times than I can count on two hands and I haven't had the gumption to go back during the Summer since around the time MGM opened in the late 80's. UGH. I just don't want my magic melted and stomped on by strangers dripping with sweat.
Just my .02 cents