I had a nurse bring up the suggestion of a pap smear not too long ago. I giggled a little, and then disclosed my trans status. He was just fine about it... we had a good conversation, and he learned a bit. When I can help it, I will disclose my trans status in a situation where someone needs to know. Men and women are different, and trans individuals are different still, so in many medical scenarios, letting the staff know about that aspect of you would be important.
For cases where male/female/trans differences don't apply, however, I would not be very likely do disclose.