If there is nobody, or if it's closed enough (not with large gaps between the ground and the door...). Best is well closed restrooms, with real walls and not just wood planks, hidden from everybody, or that people don't know about. Plus, there generally is paper and soap where people don't know about, and it's clean. I just found some like this in my university not long ago, funnily enough.
If I really have to, I'll go anyway... If not I'll wait for a good place or home.
I never use urinals.