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.
The fade on my hair keeps getting higher, and the fade on my beard lower as the grey is creeping in both directions. I'm glad the "barba italiana" look is in. I have no visible grey right after getting my hair and beard cut, and I can usually go 3-4 days before any grey starts to be visible again. Then I age 10 years in two weeks, only to have the clock turned back after a visit to the barber.