Most experts agree that the system of tipping has it’s problems, and studies have shown that tipping foments everything from racial discrimination to sexual harassment. In a perfect world, employers would all pay their employees a fair wage, and tipping would not exist.
But we live in an imperfect world. Many American workers rely largely on tips as a source of income. Only 7 states require that tipped workers be paid minimum wage. Regardless of how bad a system tipping may be, until we replace it the only ethical thing to do is tip.
The short answer is, 15-20% will usually land you in the right place. The long answer is, it depends on the service.
When it comes to food, we all know 15-20% is customary for waiters and bartenders, but 5-10% is fine if you’re eating at a buffet and serving your own food. Likewise $3-10 is a reasonable tip for a delivery driver (higher for long distances or bad weather), but if you’re getting take-out $1-2 will suffice.
In fact, $3-10 is a reasonable tip for pretty much any delivery, ranging from flowers on the low side to refrigerators on the high side. This even holds true for people "delivering" bags for you, such as supermarket baggers bringing purchases out to your car ($1 per bag), or valets who handle your luggage for you ($5). If a valet doesn’t touch your luggage but just gets the car, $2 is enough—unless you drive the kind of car that says you ought to be tipping $5.
Speaking of your car, you don’t need to tip the gas pumper at full service stations unless they do more than pump gas. If they wash your windows or check your fluids, $1-2 is appropriate. Meanwhile, if a tow truck shows up to get you out of a jam—be it locking yourself out, a jump-start, changing a tire, or a tow—$5 is an excellent way to show your appreciation in a time of need.
When it comes to travel, tour guides should be tipped a few dollars per person in your group. For hotel maids, $2-5 is a good amount to leave in the housekeeping envelope. A concierge doesn’t need a tip for standard recommendations, but if they hustled to get you tickets to a sold-out show, $10 serves as an excellent thank you.
And while most professionals do not expect tips, for anyone who provides a service for you year-round, from superintendent to mail carrier, $20 or a small gift around the holidays will always be appreciated.