It depends.For some parts of the session, the customer has to be accountable. If you're cold and didn't say anything, shame on you. If massage is not strong enough and you didn't say anything, shame on you.
If they put forth the effort, but the massage was poor, I still tip 10. If good massage, maybe 20 or more depending on various other circumstances. I.e. if the massage was discounted to start with, I may be slightly more generous...but there has to be effort..a lazy effort results in small tip..