I tip generously - I visit Argentina on holiday, sometimes on an extended basis (my wife is Argentinean). I'd tend to leave around 20% depending on the service, how much effort they put into understanding my rubbish Spanish and the overall price of the meal.
I got told off by my sister in law because the taxi came to 2,200 pesos, I gave him 3,000. My logic was that the overall cost of the taxi was very cheap (compared to what I'm used to), bloke seemed nice, 800 pesos might mean something to him but very little to me.