I was at WDW last year from 6/26-7/5. The crowds at the beginning of the trip were normal for the summer, but they got worse and worse as July 4th came closer. As long as you're not in the parks the first few days of July, it shouldn't be too bad. Just go in early, use fast pass, and drink lots of water!!
For those of you planning on visiting a park on 7/4- I highly suggest you make other plans. MK was the most crowded I have ever seen it (and I'm used to summer crowds) and it was just unbearable. I suggest you spend the day by the pool at your hotel, go resort hopping, or shop at DTD. There are plenty of places (I hear) that you can see the fireworks from. In my opinion, the fireworks just aren't worth those crowds! TO give you an idea-- we watched the fireworks at the end of Main Street, near the entrance to tomorrow land... It took AN HOUR to get to the front gate!!!
Have fun, and like I said, use fast pass and you'll be fine! I'm jealous, I'm not going back until next May!
Melissa