This is unfounded fearmongering. I tended bar in America for years. Bartenders in general have a high tolerance for jerks. You'd have to be quite a pr1ck to cause one to spit in your drink, and it takes more effort to dilute a drink or pour a short shot in many cases than not. Tipping as-you-go may buy you a little more attention from the barkeep, but plenty of people put it in their payout, too.

The best way is to tip on the 1st drink to make sure he'll keep an eye out for you later on, then add whatever you think is appropriate when you finally pay out.

That said, I had some pretty diluted and crappy Whiskey con Limons in "good ol'" Barcelona, and I'm a fine tipper. I wonder what that bartender's excuse was...?