Early on in my career, I was taught to leave it off if I want extras and cover it if I don't, I have been doing that ever since. This way, no awkward moment for either side, if she walks in and does not want to provide more, she simply cover you up and then you know. If the massage is uncovered and there is 15 min. left and no hint of her asking, I will ask, usually ladies ask right at the 15 min. mark.