Like most of the ladies have stated here, I go into the bathroom that is appropriate for the way I am dressed. I can't imagine going into the men's room pulling up my skirt and taking a pee in a urinal, although I do find the visual amusing. I have never been called out for being in the ladies room, and I do agree with the others that said it is a matter of attitude, not necessarily how passable you are.
In fact, I was out with some girlfriends one night and had to use the bathroom in the club we were at. The band was a TG band, so there were many t-girls there as well as our group. Since there were also drunk guys, I obviously needed to use the ladies room. Once inside the ladies room, I started chatting with a couple of younger GGs that had little exposure to TGs. We chatted for a few minutes, although one of the young ladies was already over the limit and she could not believe that I was not a GG. In fact, before exiting the bathroom, she loudly insisted I had to be a real woman and her friend had to quiet her down. I had to laugh, and I also thanked them for the chat and what I considered a compliment.