Feeling grumpy about that extra 15 (or 20) percent? Bruni makes a direct and concise case about why tipping is an inherent part of dining out, and not an option to be weighed on the diner's part. If restaurants raised their waiters' salaries, to take the onus of tipping off of the diner, the increased salary costs would be applied to the bill in other ways. "This probably is a zero-sum game," says Bruni. He agrees that the socially acceptable percentage seems to have crept up over the years, but concludes: "Until that system is changed, skimping on the tip—unless you’re intentionally rendering a warranted verdict on bad service—is a cheat of sorts: it’s a violation of an implicit contract."