The following is an actual (handwritten) sign that hangs on the wall of All You Knead, a Haight-Ashbury restaurant.
"Attention, Foreign Travelers:
A quick guide to the wacky American custom of TIPPING.
20 percent -- great tip, great service
17 percent, good tip, good service
15 percent, fair tip, fair service
10 percent: another way of saying to your server, 'you suck and I hate you.''