Well, for me, there are two aspects: What my girl friends told me, and what we do during steth play sessions.
One of my early girl friends told me that she always had to remove her bra for auscultation. The doctor told her right at the beginning of the exam to strip her upper body ("Mach Dich oben frei..."), which meant to take off her bra, too (she had 75B european size - btw: I've still got a bra of her!). She even told me, that the doctor put his stethoscope (unfortunately, she couldn't remember make and details; she just knew that it was red and cold ;-) right onto her left nipple! She liked to tell me about auscultation, but I don't know if she liked to be stethed by the doctor. As I had already my KaWe Multiphon, I once took all my courage after talking about stethoscope exams, and I asked her if she will allow me to auscultate her. First, she was surprized that I had a stethoscope, she laughed; then, she aollowed me to listen to her heart. She was about 18 yo and had a very sexy third heart sound! I had a tremendous erection when I stethed her. Which she noticed of course, and she was astound about my reaction! So, stethoscope games became part of our sexual live. We always played the scene at her doc: "Please strip to the waist and remove your bra..."
One night when we were out in a bar together with her 16yo sister (70C euro bra size - I've stolen a bra from her which I still have - and "use" ;-), we started the topic of medical exams and auscultation, of course to make little sister to tell us about it. It didn't work, she was to shy and became red. So, we continued, and my gf told about the stethoscope on her nipple. And little sister agreed "yes, me too..." Both girls had the same family doc. So, 16 yo busty Rita also had to remove her bra for the stethoscope. During the whole conversation, I had a severe hard on and a lot of pre cum on my cock. We had a good stethoscope fuck afterwards - without Rita, unfortunately :-(
Other women I have asked, didn't either like to talk about medical exams, or they didn't give me information whether stething with or without bra.
In our stethplay sessions, there is auscultation with bra in the beginning and afterwards stethoscope without bra. And taking off the bra is kind of a "ceremony". Especially, when it's the first time, we have a new patient. It's always arousing to see her naked breasts for the first time. And then put the cold stethoscope on her hot skin! I love that.
So, for me as a male, I can report experience of women I know and I can say, that we love stething with and without a brassière. Which sounds logical for a stethoscope and brassière fetishist, doesn't it ;-)
RedStethGuy alias Steth(oscope and) Bra(ssière) Lover