It doesn’t matter what clothes you show up in, Kurono says, as most spas will have you undress and slip into a robe. (At Tomoko, every client receives traditional Japanese yukata clothing to wear during her visit.)
As for the potentially awkward question of how much to disrobe during a body treatment, Kurono says it’s all up to the client. It’s fine to take off undergarments, but for the more modest, the spa provides disposables to wear during treatment.
If you’re on the fence, going underwear-only is a good default. Bras can get in the way during a back massage or any skin treatment that involves the decolleté.