Got an Answer?
I'm a massage therapist, and my clients' breasts are never exposed. If I'm working on their abdomen or upper chest area, there's a hand towel covering the breasts still. The only situation I would think it's appropriate would be a particular combined physio/LC who actually works on breasts (usually with ultrasound) - in which case shielding them would defeat the purpose anyway.
I have been seeing an osteo for a year for my back and chest, when laying on my tummy he just unclips my bra and when I have to turn over if I need to take my bra off he leaves the room. And I put a towel over my breasts. All depends on what style bra I wear.