Suddenly that seamless Uber service has entered the murky realm of when and how much to tip, a realm that many travelers would just as soon avoid as they try to navigate themselves through new places. A Harvard Business Review article called it a “ ‘Don’t, but you probably should’ tipping policy”— and a mistake.

One reason for that, writes Rafi Mohammed, a pricing strategy consultant, is that it could hurt riders. After all, they are rated by Uber drivers. Customers with low ratings are less likely to be picked up when they request a ride. “In Uber’s case, drivers will know whether and how much you tip before they rate you,” Mr. Mohammed said. “As a result, tipping is now a Tony Soprano-like veiled threat: ‘Pay up or I’ll give you a poor rating.’ ” (He points out that drivers for Lyft, in contrast, rate passengers before knowing whether they tipped.) Acknowledging that it’s unlikely Uber will forbid tipping, he suggests that the company adopt Lyft’s model, which allows riders to tip within the app (now putting Uber at a competitive disadvantage, he said).

So what’s a traveler to do? Start carrying cash for Uber tips? Budget for more expensive rides? And if you’re traveling on business, do you ask for a second receipt just for the tip (since the Uber receipt for the trip itself is automatically pinged to your email inbox)?

I used Uber three times in San Francisco and Berkeley in recent days and none of my drivers asked for tips or had signs in their cars. It was the usual seamless experience. But what if there had been a sign? An opinion column in Bloomberg View this month suggests Uber riders skip the tip. When the writer, Virginia Postrel, took an Uber to Los Angeles International Airport, she said she saw a handwritten sign on the back of the passenger seat: “Tips — Appreciated — Thank You.”

“Although my driver was fine and I’m generally a good tipper, I resisted the instinct to comply,” she wrote. “He got a five-star rating but nothing further — not because I’d begrudge him the extra money, but because the only way to preserve the frictionless Uber experience is to defy the social pressure to tip.”

Uber is not the only company that has told riders that tipping is unnecessary. In New York City and Chicago some residents have been pleasantly surprised by the affordability of the ride-sharing app Via, which has the tagline “ride for the price of a latte.” In New York, it can be even cheaper. Weekday rides from 6 a.m. to 9 p.m. in Manhattan south of 110th street are $5 plus tax; and $5.95 plus tax from 9 p.m. to midnight and on Saturdays. When you search for the tipping policy, you find this (which has some of the same language as Uber): “One of the things that makes Via special is the all-inclusive fare. We never want you to pay more than that fare for a ride, so although our drivers are the best in the biz, a tip is neither required nor expected.”