Yes, this is my understanding and has been for quite some time. There is no additional obligation to tip beyond what WoH provides.

I had always known this from having read about this from some very some fellow FT'ers a while ago. I've always just written "Globalist" on the slip's tip section, and will generally add some cash on top of it, unless the service is outright atrocious. To each his/her own and not wanting to veer into the American tipping culture debate, I'm from the school of thought where "if you take care of me, I'll take care of you."