An interesting (and wonderfully brief) paper in the Journal of Applied Social Psychology (here) reports results from a study of restaurant customer tipping. Although the study and data are limited, evidently tipping "guidelines" on bills serve as a cognitive anchor with the effect of exerting upward pressure on customer tipping behavior. The abstract follows.
"This study examined the role of gratuity guidelines on tipping behavior in restaurants. When diners were finished with their meals, they were given checks that either did or did not include calculated examples informing them what various percentages of their bill would amount to. Results indicated that parties who received the gratuity examples left significantly higher tips than did those receiving no examples. These results and their implications are discussed."