Lordy, tipping in the US just gets wearying. As a Brit, it is also a source of enormous anxiety.
Even on the plane, I start trying to work out how much change I'm going to need for the tip-a-thon that occurs between landing and hotel room. Then usually I have to make a special stop a Travelex so as to get enough change so as not to be left stranded in a situation where a tip is expected and I don't have change.
Then for the remainder of the trip - particularly if you are staying somewhere upscale and are using the hotel facilities (pool, spa etc.) - you spend the whole holiday haemorrhaging dollar bills.
Let us not move towards such a system here, and say "NO" to tipping in the MB spa.