The service charge is not optional. Scrub and MC automatically add 20% to the bill. By law, it gets divided up between back and front of house people. The waitresses are making less since this system was implemented (I asked).

What irks me is the hostesses get a cut, and there is one lazy, difficult one at Scrub that I hate to see get our money. She always tries to give us the worst table in the place, and she's too lazy to even refill water glasses.