There are a few situations when leaving that line on the check blank should be guilt-free:

-At an all-inclusive resort
All-inclusive means all-inclusive, even if you spend an entire week getting pampered at the spa. Don't feel bad about failing to slip your masseuse a $20 -- all-inclusive resorts add an 18 percent gratuity charge to your bill at the end of your stay.

-When it's not their policy to accept tips
Phillips, who began her career as the owner of a spa that didn't accept tips, says that spas do so in order to provide a stress-free experience to the client. "If you've had multiple services, and you're a little spa drunk at the end, all of a sudden now you're doing math and it ends the service on a different note," she explains. Before you book the service, call ahead and ask what the tipping policy is.

-When you had a bad experience
For spa and salon owners, the bottom line isn't "how much of a tip did I make today?" Instead, they want repeat customers who bring in referrals. If you didn't have a good experience, you don't have to tip. You should, however, ask to speak to the manager or owner instead of leaving a passive-aggressive $0.50 tip to express your feelings. Phillips says that service providers are apt to think that a client is cheap if they leave a bad tip, rather than it being a reflection on their service.

-When they're fixing a mistake
Sometimes, a haircut or color doesn't turn out the way a client envisions, and the hairdresser will offer to fix it. In this scenario, Fontana says it's okay not to tip; In fact, he will refuse tips if someone requests a revision. "If the client has to go back, it's because they don't like it and you didn't do your job properly in the beginning, so you should not take any more money from that person," says Fontana.

-The ambiance wasn't up to standard
If the person working on you was doing a great job, but the atmosphere wasn't ideal, it's not their fault. The person to address those concerns with is the owner or manager, not the service provider. "You don't get a refund on your airline seat if there was a crying baby. If the service provider was good, tip them," says Phillips.

If you do want to express how much you omfg-love your new hair, Phillips says the best way isn't always through a tip. "One of our technicians just had a baby, and a client brought in a special baby gift for her. When a client brings in something like that, it's way more meaningful than an extra five bucks on a tip."

If you hand a bartender a $20 tip at the beginning of the night, it's likely that he or she will take care of you for the rest of the night. Likewise, tipping well at a salon can have a similar effect. As Phillips puts it, "It's human nature to want to do something extra for someone who has done something for you." She says this doesn't translate to free services, but it does mean that things like fixing chipped polish or using an extra product in a massage come free of charge.