If you're unhappy with the service you've received, is it ever okay not to tip?

 No. You should never let your money talk for you. If you get good service, in addition to leaving a good tip, you would want to thank your server, bellboy, etc. When it goes the other way, you still should leave the customary 15 percent. If you had horrendous service and it was the service provider's fault, some people might go as low as 10 percent. But we suggest that you leave 15 percent and then immediately speak to a manager to express your dissatisfaction. Say that you're unhappy with how you were treated and that you're reluctant to return after such an experience. That will speak volumes to a manager.