I'm more or less a reflex tipper. I usually tip in cash, even when putting a meal on plastic. (It seems more "real" to me that way. Okay, I'm weird.) I double the sales tax, round up to the nearest dollar, and done. I rarely, if ever, think about it. Unless the service was poor. Then I wind up in a quandary. What IS an appropriate tip when the service was something between atrocious and craptastic? When this question comes up, with friends and strangers alike, it seems to always spark an argument.
Those who were once waitstaff often argue that tips shouldn't be tied to perceived quality of service - since restaurant wages are so low, tips should come with the territory. Cross them on this, and they tend to become rather angry. Not always, but that's the way to bet. Of those who have never been in the business, there are a few who argue the "always tip" line, but they aren't generally as militant about it as those who (as John McG pointed out) once had their rent money riding on it. And every so often you come across someone who really emphasizes the idea of perceived quality of service. That becomes a hard row to hoe. The idea that there is a standard of service out there that we as customers are not fit to judge, but only to pay for, doesn't usually go over well.
There's usually a small group of people who advocate leaving a minuscule tip, such as a nickel, or a quarter, as a deliberate insult, and a way of demonstrating their displeasure. No sooner have the words passed their lips when Hey, Presto! the militant always-tippers are calling for the Crosses and Nails to be brought out, and the Scourges reset with freshly-broken glass. I've never seen it escalate to violence, but I have seen a few friendships founder, catch fire, and sink into the swamp. Not pretty. The idea, I'm told, is to make it very clear to the server that the service was less than was desired. It's a position that almost always winds up on the defensive. Mainly, I think, because it does add insult to injury, and is thus intentionally mean-spirited. Which I suppose is the point, but that doesn't really make it any more palatable.
From time to time, I've encountered people who tip on a sliding scale. They rate the service against their expectations, and tip fractionally, in accordance with the rating that they've come up with. You'd think it would be a workable middle-ground, but most people don't go for it - they don't want to be bothered with trying to come up with a "fair" calculation, it seems that no-one can ever agree on what fraction of the overall expectations were met, and there's usually at least one person who seems dead-set on demonstrating why the rating should be 100%. So the fractional tippers normally practice their art only when they're out by themselves.
I commonly fall in a last group - those that advocate for not leaving a tip at all, if the service isn't up to snuff. I was brought up with the idea that tips are meant to be a reward, and I tend to tip anytime someone does me a service that I wasn't expecting, or does a better job than I would have expected of them. No worthwhile service then, equals no reward. I suspect that it has a lot to do with the fact that you pay for many services before you get them, and if you don't get what was promised, you're still out every penny you paid. Therefore, when I have a chance to not pay for something that I wasn't given, I jump at it. But it does seem unfair, somehow, to make it an all-or-nothing proposition. But for some odd reason, no tip at all seems less mean than a partial tip. Even I don't pretend to understand the logic behind that line of reasoning, but it's there nonetheless, and I can usually find someone else who agrees with me.
It's a never-ending debate. I try to avoid sparking it myself - fights that you can't win don't seem to be worth starting, but I am always interested to hear people's rationales for the positions that they stake out. But in the end, I still haven't made up my mind which is the "right" way, even though I've settled into a pattern of behavior. I still find myself grappling with the pros and cons every time the situation comes up, even if the outcome is always the same.
And in case the title of this post makes no sense to you...The Clampetts, along with Mister Drysdale and Miss Hathaway, are in a posh Beverly Hills jewelry store (IIRC). The young man who's been helping them stands by expectantly, with his hand out.
My father still finds this uproariously funny, and he quotes it every chance he gets. I have a vague recollection of having seen this episode myself, but that was a very long time ago.
Mr: Drysdale: He wants a tip.
Jed Clampett: Oh, a tip? (To the young man.) Plant your corn early in the Spring.