Just got back on Sunday, had lovely weather the whole trip!
The restrooms are clearly marked on the park guide maps, so you will not have a problem finding them. When we went to MK and Epcot, the parks were pretty crowded, but the restrooms still had no lines; no waiting, as the facilities are very large.
The only place there was a line to use the ladies' was in a ts restaurant, so I would not go in just to use their bathroom if you are not eating there, if I were you. I also think it is not appropriate to do that, IMO.
Trust me; with my DD we took a lot of bathroom breaks and had no problems.
(Of course, there was the problem when I went back to work on Monday after our trip and expected the toilets to flush by themselves, LOL!!)