Get pesos as soon as you arrive. Based on my experience you will use more local currency than i have used in any non-USA country. With all of the theft/robbery warnings we had planned to use our credit cards to minimize our losses. Here were our eperiences.
We booked a tour through the hotel for our first full day. After the tour ended and they wanted to be paid, they said it had to be in cash/pesos. Well i only had US cash in the amount needed, which they finally took after some discussion. We did a very expensive tango show dinner that night ($130US per person for transportation, dinner with a bottle of wine and show), and fortunately they did take credit cards.
The next day we thought we would take the hop-on and Hop-off bus, as there were a few places i wanted to go back to. This tour bus company will only take pesos (no credit cards and no US).
Then at the Havana store, where they sell the national treat, the guy behind the counter wanted to see my passport if i used credit.
At that point, i was ready to leave BA...so we went to Iguazu Falls. At Iguazu Falls Airport, the cab drivers will only take pesos (no CC) plus you have to pay the park entrance in pesos ($110 pesos + $130 pesos). However once in the park CCs are readily taken. When we returned to BA for an overnight before flying to Peru, we stayed at the new Holiday Inn by the airport. The cabs going to and from there will only take pesos....
We were out of pesos and patience at the lack of credit cards in light of all of the theft warnings. I am posting this not so that others can learn from that experience, and get pesos and lots of them as soon as you get to BA... You will need them.