I really wouldn't worry about it. I've been there at the busiest times of the year and have never had to wait for a bathroom. (And BTW I am queen of the bathrooms according to my children. Even I will admit I am in there a lot!!LOL!!). Seriously most WDW bathrooms are huge!! Sometimes there looks like there is a line, but in actuality women are just standing around. I always go to the farthest stall, usually very clean and almost always empty. The only one I can think of off the top of my head to avoid is the one in the Land upstairs near Garden Grill, way too small, try downstairs at the food court. Also the ones in Wonders of Life in Epcot are way too small too, if there is a line it will be there. Try goiong before you go into that pavillion. Woman are usually good about letting you cut. I always let pregnant women and small children ahead of me if it looks like they are in a hurry. Oh I just remembered something else, the bathrooms at the Crystal Palace are too small also. If it is crowded, you may have a wait. Also Tony's Town Square, just go across the square, there is a HUGE bathroom there.
Geez!! I better stop!! Maybe I should write a Disney Bathroom book!!! LOL!!!
Have a wonderful vacation!!!