A waiter once approached me in a club in Washington DC and held out a $100 dollar bill. ‘Your father’, he said, ‘gave me this as a tip’. My father was, you see, a bit worse for the weather after a few glasses of bourbon (and after all, all dollar bills look the same). I snatched the hundred dollars from the honest server and gave him a five-dollar bill instead.
Let that be a lesson for him.
Tipping in the USA is necessary because the wait-staff only get as little as $2.13 an hour (a preposterous amount, and apparently frozen since 1991) and they must make up the rest of their take-home earnings with tips. These are generally accepted as running at 15 to 20%. Even the credit card receipt asks you how much to add. Everyone over there seems happy enough to pay this.
In Spain, where we foreigners often wonder about the intricacies of tipping, the rules are different. To start with, the staff should be getting 10€ an hour from the owner, plus social security and time off for the odd smoke around the back. Many of the employees evidently are listed as part-time by the employer even if they’re not. Since tipping is a voluntary act rather than an institution, we sometimes do, and then again, we sometimes don’t. After all, apart from taking a photo of us on our smartphone, a waiter's main job is to stop us going into the kitchen and getting our own plate of chicken and rice off the cook.
The cheaper places don’t seem to show much interest since they’ll pass you the change from their hand to yours along with a friendly hasta la próxima – see you soon. Others, a bit more on the ball, will return your change on a small saucer. You are left with the options of either leaving it bare as you get up to go or else decorated with a bit of calderilla – pocket change. It’s never going to be more than five per cent or so.
No wait, I’m being told by an upmarket Spanish magazine that one should leave ten per cent, unless they’ve already added a gratuity to the bill. Fancy places huh!
Of course, some waiters make a decent amount of cash on the side – and no doubt neglect to declare it to the tax-man.
An article at Wiki regarding tipping in Spain says that ‘In 2007 the Minister of Economy, Pedro Solbes, blamed excessive tipping for the increase in the inflation rate’. Quite a claim! 
But let us drop into a bar in Madrid for a beer and a sardine. There, a new campaign has been launched by the conservative regional leader Isabel Díaz Ayuso, to encourage tipping – as (we read), she doesn’t want to raise the minimum wage. There’s a TV campaign to encourage the practice – a few extra coins could help the waitress pay for piano lessons for Elenita, or maybe English classes for Roberto… This remarkable commercial, called #YoDejoPropina, can be found at YouTube.
Useful if you mistakenly leave them a hundred euros…