Tess Vigeland: How much do you tip? 15 percent? 20? 25? What started out as a way to reward good service is now pretty much a given. You tip even if the wait staff pours an entire bottle of wine on your head. So if you're gonna tip no matter what, at least you've got all kinds of gadgets to help you do the math these days. But in some ways, technology has made tipping even more confusing than ever.

Here's Marketplace's Jeff Tyler.

Jeff Tyler: Let me sketch out a couple recent experiences where the credit card receipt presented me with tipping conundrums.

One happened at lunch in a deli here in Los Angeles. I made myself a salad at the salad bar and took it togo.

Cashier: Hi, sir. How are you?

Tyler: Good. How are you?

Cashier: Good, thank you. $6.50.

The credit card receipt had a blank space next to the word "TIP." Now, the person behind the counter didn't do any more work for me
than the staff at Taco Bell. So, should I leave a tip?

Here's another example: Last fall, I visited my neighborhood sushi restaurant.

Restaurant staff says "welcome" in Japanese

When the bill came, it had a tip guide printed across the bottom. I paid with credit card. When the credit card receipt arrived, it also had a tip guide. Both suggested a tip of 20 percent or 25 percent. But the corresponding dollar amounts were different.

Why? You may have guessed: One receipt calculated the tip after sales tax. I spoke to waiter Jeff Lee about the discrepancy. Though he's on the receiving end, Lee says he doesn't really care if customers tip before or after tax.

Jeff Lee: It wouldn't really be like a significant difference. Maybe like the difference of a couple cents. Maybe like, not even a dollar, most of the time.

Apparently, other customers here are equally indifferent.

Tyler: Have other consumers said anything about that?

Lee: Actually, you're the only one who's brought that issue up.

It might be unfair to make Lee the arbiter of a tip question. He grew up in South Korea.

Lee: In South Korea, there is no tip. They're offended, basically, if you tip them.

I expected the management here might be concerned about offending me with its conflicting and inflated tip guides. At the least, I thought they'd fix the discrepancy. I was wrong.

Lee: When I spoke with the head office, they said there's nothing they can do about it. It's up to the credit card machines.

Let's say good-bye to the sushi bar...

Man: Arigato gosaimasu!

...and stop by a fondue restaurant called The Melting Pot. The waiters' job here is labor intensive. They make fondue at your table.

Waiter: Alright, we're going to add a little cilantro here. Garlic. We like or we love?

Laurie: Love.

Waiter: Good answer. I don't trust people who don't like garlic.

The bill here also has a tip guide. The suggested rates range from 15 percent up to 25 percent. I asked the restaurant's director of operations, Dana Robertson, should people feel guilty about leaving less than 20 percent tip?

Dana Robertson: They should tip what they feel their service was. So, I mean, customary tip now days in restaurants I would say is between 15 and 18 percent.

National studies show the average tip is 18 percent. But service may not have much to do with it.

Mike Lynn is a professor at the School of Hotel Administration at Cornell University. He points to studies showing that the weather can be as influential on tips.

Mike Lynn: If it's sunny outside, you tip more. And that relationship between weather and tipping is as strong as the relationship between service and tipping.

If service isn't a significant variable, does that mean that I should tip the same rate for alcohol?

Lynn: Etiquette experts suggest that you're supposed to tip on the total bill, and that includes alcohol, wine -- all of it. Except, not taxes.

Hear that, sushi restaurant? No tipping on the tax. But in practice, many people do anyway. I asked Lynn how we ended up with this vague financial convention. No one knows for sure, he says. But there are some interesting theories.

Lynn: George Foster, who's a retired anthropologist from the University of Berkeley has argued that tipping originated in eating and drinking places as a way to prevent the workers of those places from being envious of you as a customer.

There is some evidence to support this.

Lynn: The word "tip" in many, many different languages around the world, translates to "drink money" or "money for drink." And in some Asian countries, "money for tea."

In terms of modern tipping conventions, I wondered about tips on to-go orders. Lynn says it's becoming more common. But...

Lynn: I think it's crazy and I don't do it.

At the deli where I take my salad to-go, I feel bad if I leave the tip space blank. I assumed other customers might have a similarly guilty reaction. So I expected people using credit cards would tip more than those who paid in cash. Wrong again.

Jack Elisha is the general manager of Trimana.

Jack Elisha: We receive ten times more cash as a tip in the tip jar.

Customers will pay for a $7.50 with a 10 spot and deposit the change in the tip jar. He says it happens five or six times a day.

That's one limit on technology: Credit cards don't leave you with an annoying pocket full of change just waiting for a tip jar.

In Los Angeles, I'm Jeff Tyler for Marketplace.

Vigeland: OK so really, what do you do about tips? We want to know. You can post on our Facebook page. There you'll also find responses to that question from people across the country.