Any time in the BW/Q is better than not going IMHO.  That being said, the best time for us involves taking advantage of a holiday to give us the extra day in the park.  Since Memorial Day and Labor Day also are getting more toward the shoulder seasons, they take top billing in my mind.  Just don't expect to easily find campsites within a 4 hour paddle of an entry point (BW and southern Q) on the holiday weekends and plan so that you are in the middle of the park at that time.