The difference between 15% and 20% on a $15 bill is 75 cents. The difference between pre- and post-tax is even smaller. You might not get bonus points for ignoring that, but you'll definitely lose points (in my eyes, at least) for fretting about such a small amount of money.
Anyone loses points (in my eyes) for tipping an exact amount regardless of the quality of service. It's a tip, it's supposed to reflect how well you were treated. If you do 18 or 20% no matter what, you're missing the point of tipping. If I'm eating spicy food and my glass is empty for the entire duration of my meal because the server is not paying attention, you're lucky to get a tip at all. I've left 50% tips, I've left no tip, and I've rounded $9.99 to $10 and called that a tip.
Anyone working in a restaurant expecting a tip for bare minimum service has my pity, but not my money. You get my money by doing a good job.
It's a tip, it's supposed to reflect how well you were treated.
That may be how the practice started, but that's no longer how it works. I mean, you can clearly tip however little or much you'd like, but, culturally, the tip is nothing more than a formality. And like any formality, you're free to ignore it, but you should be aware that that's what you're doing.
The idea of a tip is to compensate service based on quality. If that's not the goal, then the cost should just be integrated into the price of the meal. Since it's not, that means tipping should be reserved for compensating service based on quality. Restaurants are free to set service fees (and many do, "A fee of 18% will be imposed upon parties of 10 or more"), but a tip is still a tip.
You give me poor service, you get a poor or nonexistent tip. If the server doesn't like that, then they should provide better service. If the restaurant doesn't like paying their staff to make up the difference between what their wages and minimum wage, they're free to take action against that server (from more training to just firing them). That's the behavior that tips are meant to encourage. Either you're a good server and you make more than minimum wage, or you're a poor server and you don't have a job anymore. That's how the entire economy works.
At any rate, a tip is not a formality. If people are treating it as such, I have a problem with that. Either do away with tips and pay the staff what they're worth or make tips based on quality of service.
Of course and I said as much. Whether or not to shake the hand of someone you meet is also up to the hand-shaker, but, again, you should just be aware that declining to do so bucks convention. You're talking about what you think tipping should be; I'm talking about what it is.
