In Manhattan, I double the tax for normal service, and tip 20% or more for extraordinary service (DH and I have tipped as much as 40%, in cash, to a waiter who really gave outstanding service; we also told his manager that our evening had been a delight in large part thanks to the waitstaff, and this waiter in particular), all based on pre-tax amounts. I don't think you should tip on the tax, but I could be wrong.