But, were I to get a full body massage, would I keep the panties on or take them off?
I take mine off and lay face-down as I await the masseur or masseuse. On entering the room, he or she will take a towel and cover whatever they want temporarily covered. Later, they'll move the towel elsewhere whenever they start working on a currently-covered place.
Full disclosure: This may
be one of those situations where I inadvertently commit a gaffe, being a European and much more comfortable with nudity than the average American seems to be. The masseur or masseuse, in turn, may
be too deeply invested in a customer-is-king mindset to say something. As best I can tell from body language and mimics, though, that doesn't seem to be the case. Nevertheless, you may want to take my account with a grain of salt.