I used the metro/"subte" in Buenos Aires and found no reason to avoid it (or avoiding certain stations). It was cheap and easy to use. Taking a taxi was interesting in that I remember the car was a jalopy and the driver threw all our suitcases in the front seat (kind of odd but who knows, maybe his trunk was already full). It was not possible to hail one off the street, they would just drive by even if empty. There is a decent chance that a taxi driver will pretend you gave them a different bill when giving you change back (so look at it carefully and even learn how to say the number in Spanish), and some can be quite aggressive (the ones manning the airport). As far as safety goes, I think it's worth remembering the mundane - not using your seat belt is probably the highest risk to your safety you'll experience and yet so many people just don't do it once they get in a cab. I never felt unsafe in BA or (northwest) Argentina and I also wouldn't avoid a poor, but colorful/ touristy neighborhood like La Boca during the day.
One major piece of advice I could give about BA is that you may experience unusual pains in getting small change/coins. If you try to give a cab driver a big bill, they likely won't be able to break it. There is a real hard currency issue in Argentina and high levels of inflation. Everyone would love to get (your) US dollars instead of payment in Argentine pesos if given a chance since those are actually worth something and they make a profit in the exchange.