I was told by everyone I asked that I should not tip the taxi drivers, because it's not customary for the locals. My first cab ride from the airport to the hotel I pull out pesos for the payment and had some USD in my other hand and the driver says to me: And you'll also give me 3 US Dollars for tip!

He did this with such a big happy smile, I was kind of shocked and amused so I gave him 3 bucks tip.