While it is important to be cautious and aware, I think the tourist guides are a bit too strong on harping about dangers of hailing cabs. As long as you trust your instincts and are comfortable in cities like New York, should have no problems hailing a cab and taking it. It is also a matter of convenience - at the hotel, it is easy to get a radio taxi called in. But after dinner, after a walk around in the neighbouhood, it makes no sense to worry about this and call in for a cab when you can just do what the locals do, hail a cab on the street and not worry about this too much. And, do be careful about large bills since change is hard to get, and there are reports of counterfeited bills - best to have exact change or small bills to pay for the ride, around the Microcentro and neighbouring barrios, tabs should run around AR$10, so have AR$10 bills and change with you. Taxis in Buenos Aires are a very good deal, just under AR$1 per kilometer in 2008.