Paula, this is one of the Eternal Questions.
My philosophy is that you don't give anyone reason to notice or worry about which restroom you use. You use the restroom of the gender you are presenting, but you don't loiter there longer than you need to in order to do your business, wash your hands, and exit. It's best to use the restroom right before you intend to leave an establishment. If some cretin takes offense he or she can argue with your back as you walk out the door.
