I use McDonalds bathrooms in a pinch quite often. I don't ask or anything, I walk in, it's there, nobody cares. However, it depends very much on location, and I do this only in suburban areas. if you go into Manhattan, bathrooms that are easy to spot are very much locked down, if there is even one at all, so you have to try to find less obvious bathrooms that don't attract a lot of attention from street traffic.