We all know that scams are common in Argentina. Concerning taxi drivers I must say that I found them extremely honest. The only case where I really got ripped of was when I was quite drunk. A few (very few) times they did not take the most direct route, but never more than 5 cuadras (you know how it is, miss a turn and the next is one-way, etc.). A few even stopped the meter the moment that happened. I NEVER EVER got a fake bill!
but there are two things:
1.) I know the streets very well. If they asked me something about the way to get where I want to, I generally was able to tell them.
2.) I never used large bills. Of course if the meter says 75 I pay with a 100. But if it says 65 I use the smaller bills. In a few exceptions where I was unable to, I already asked before getting on the taxi, if he has change for a 100. Those who agreed never tried anything funny.
3.) I generally spoke to the driver. I don't know if it makes a difference and I did not do it to reduce chances of being ripped off. I just like to talk to people. Anyway, I could imagine that a driver who considers you a nice and good person (or simply somebody who is not stupid/an easy target) is less likely to rip you off.
Apart from that, you should not leave your luggage in the taxi and search an ATM for 10 minutes once you arrive. I heard of one story, where the taxi (and luggage) were not there any more after that time. That's just plain stupid. But basically BsAs taxi drivers are very honest people. I had more attempts to rip me off at home in Austria, which is generally known as safe, honest, etc. (plus: I am a local there). Of course being a taxi driver in BsAs is actually quite a good job and they are rather proud, in Austria it is not.
Just take care, know to speak spanish, etc.. The situation is really way better than the tourist guides say.
PS: I almost never called radio taxis (I did not like the waiting time and extra cost). Nevertheless, I preferred to hail those taxis, that carried the logo/sign of a radio taxi company and were in a good shape. IMHO a driver who does not have the funds or motivation to keep his vehicle in a decent shape is more likely to rip somebody off, or worse. Furthermore I suppose that a taxi that works with a radio taxi company is more likely to be reliable.