Best and worst tippers of food-delivery workers

Kung Pao chicken or a cheeseburger? Add an appetizer, dessert or both?

Each day, customers of the GrubHub delivery service face several tough decisions like this, but perhaps the most perplexing is the final one: How much to tip?

It turns out many of them are on the same page, tipping an average of 14.1 percent, according to an analysis of the company's transactions from March 28, 2013, to March 27, 2014.

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Thirty-five states' tips averaged in the 14-percent range, a level that's within the Emily Post Institute recommendation of 10 percent to 15 percent of the bill for deliveries. GrubHub's data only include tips on cards and eBay's PayPal and exclude cash tips, which can't be tracked by the company and would likely inflate the rate somewhat.

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"While these rankings highlight some interesting trends in tipping, it's important to note that the average tip on delivery orders hovers just above 14 percent," said company spokeswoman Allie Mack. "When it comes down to it, the differences we see in average tips among cities and states aren't huge."

While the company declined to provide the number of transactions included in the data, GrubHub handled an average of 135,000 orders per day last year. Over the course of the period analyzed, it's plausible then that the data set includes tens of millions of transactions.

So who are the stingiest tippers?

Among the states, Illinois customers left the least at 13.6 percent, followed by Nevada at 13.6 percent and Arkansas at 13.7 percent.

Once the data are broken down into cities, three New York cities made the bottom 10.

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Syracuse, N.Y., tipped the lowest at 12.1 percent, followed by Ithaca, N.Y., at 12.8 percent, Champaign, Ill., at 13.1 percent and Rochester, N.Y., at 13.4 percent.

Although Mack said GrubHub couldn't draw conclusions from the data, some interesting trends did emerge.

"Budgets can be really tight for cost-conscious college students—myself included when I was in school," she said. "Knowing this, it makes more sense that many of the lowest-tipping cities happen to be college and university towns."

Among big tipping states, the Southeast had a strong showing with five making the top 10. The top three were South Carolina (16.1 percent), New Hampshire (16.0 percent) and Colorado (15.7 percent).

Interestingly, all three most generous cities were in Colorado. Boulder tipped 16.2 percent on average, followed by Denver at 15.7 percent and Fort Collins at 15.4 percent.

It's unclear why the Centennial State cities are so nice to the delivery person.

"We have no idea why that is, but a hat tip to Colorado diners for their generosity," Mack said.

—By CNBC's Katie Little.