I completely disagree with the 2 down days idea in 10 park days. Our summer trips have been 17 park days, and we are in the parks either 16 or 17 days. The only reason we do 16 sometimes is we'll do a day trip to Daytona Beach and we may not go to the parks that night. Certainly you don't want to do commando days, that will wear you out. But for us, even if it's only 3 or 4 hours, no reason not to go to a park. Our "rest" day will be sleep in late, have a relaxing breakfast, get to the parks around lunch. Stay until before dinner, then go back home and relax the rest of the night.
As for how many times to hit each park, when you have a long trip like that we like to build flexibility in. We'll "plan" (we're not planners anyway) a few days in each, then after day 6 or 7, the rest is purely freestyle. We'll decide that morning, or maybe a few days before.