We went to MK and Epcot this past Saturday. Both were mobbed. Tron and Guardians had 70-80 minute waits all day. Rides like Pirates, Jungle Cruise, Haunted Mansion, Frozen, Remy etc. were 50-60 minutes most of the day. Our group wanted to take a last trip to Tom Sawyer Island before it closed for good and every raft was shoulder-to-shoulder like taking the monorail back after fireworks. A month earlier, it was empty!
We had maybe 15-30 minute waits for Buzz, People Mover, Philharmagic, Monsters Laugh Floor, Spaceship Earth, and Soarin' (the latter just before closing time). Fireworks crowds were packed. Despite skipping fireworks, transportation back to TTC at the end of the evening was very crowded, again shoulder-to-shoulder on the ferry and standing-room-only on the monorail.
Of course, you should take your trip when it works best for you. But I would not count on low wait times right now to the point that it is the only reason to change your dates. Also keep in mind the almost daily rain storms in the afternoons/evenings over the summer but some people like that because they can clear out the late afternoon crowd. As with everything, YMMV.
Good luck and hope you enjoy your trip whenever that may be!