I would look at the schedule and see what parks have EMH and decide if you are going to utilize them if you are staying onsite, if you are not staying onsite than I would avoid those parks on those days.
Fourth of July week = Jam Packed Parks and Jam Packed WDW Resorts.
Therefore, if you are staying off-site, definitely avoid those parks that are doing Extra Magic Hours(EMH) for that day.
If you are staying on-site, you can TRY to take advantage of EMH, but BEWARE. There are approx 23000 rooms at WDW. Figure about 4 people per room. Thats potentially 92000 WDW resort guests trying to "take advantage" of EMH.
In order to make Morning EMH work, you MUST be at the park right as the gates open for the WDW resort guests. Ride/see what you can and then I suggest leaving that park early, go to the pool, take a nap and/or etc..
Evening EMH are a different story. There is no rush to get to the park. It is in all likelyhood jam packed. Non resort guests, however, can not do anything except shop. The only other thing you have to consider, besides the potential of 92000 WDW resort guests flocking to the park for Evening EMH, is HOW GOOD IS YOUR/FAMILY's ENDURANCE??? If you start visiting parks at 8am and Magic Kingdom will stay open to 2am for resort guests, can you hack it?
Oh, I forgot Disney Vacation Club(DVC). I think that is another 10000 rooms and potentially another 40000 resort guests. Darn those evil DVC'ers!