I always tip barbers (in any country) as long as they take pride in their work and don't rush. In the US, I tip always and tip more if they don't rush.
Just curious as to how much you pay for a haircut in the U.S.? I'm old enough to remember when I had to only pay $5 ! Then after a couple of years it was $20. Now it cost me $50. I do not have complicated hair; My haircut is very straightforward and takes no more than 15-20 minutes, depending on how often the guy cutting my hair gets distracted, LOL. I used to give 15-20% tip when it was a $20 cut. Now I'm only doing 10% tip, and I try go 3-4 months, sometimes longer, between cuts, which drives me crazy since I used to get my short hair cut every month.
I tip when I travel, but I always feel uneasy if I should or not. I tipped a driver in Prague last year for driving me to a city in Austria, and he was genuinely surprised by it. I also remember back in 2015 when I tipped a Burmese boat driver; He looked confused (maybe even offended?) and asked "What is this?" He didn't take the tip, and I just felt very awkward.