I usually wear whatever makes sense for the rest of my day, not for my T appointment. That said, I have dressed business / professional when angry at my T, and I have also inadvertently worn a low-cut dress because I was headed out with friends later and I was very uncomfortable in that (my T is male). He's never seemed to notice. One time he said my sandals looked comfortable, I said yes they were. I had worn them about 20 other times in his office before that, have no idea why he noticed them that day. Only time he's ever commented on my appearance in any way.