Rissack and others are correct, don't make it too complicated. When we travel to Playa, my wife and I made use of either pesos obtained from an ATM, pesos obtained from a cambio, or the US dollars we bring along from home - depending on the circumstance. Just be mindful of the current going rates.
ATMs generally offer the best rates, but be careful when entering your PIN. Sometimes US dollars are best - my wife found a purse at a store on fifth (near eighth) that allowed her to pay in US dollars at 13 pesos to the US dollar. We found several cambios exchanging at 11.3 pesos to the US dollar, including one near the intersection of First and twenty-eighth street (a Budget car rental across the stree and down from PGs) and several on Juarez between twentieth and thirtieth. If you don't mind, you can easily find a cambio with an exchange rate of 10.5 pesos or so to the US dollar. Our ATM card gave us an effective exchange rate of about 11.75 pesos to the US dollar.
If you want to, I am sure you can spend half of your vaction time trying to determine if the cambio you are at right now offers the best rate - the other option is to spend more time enjoying yourself and consider it a vacation expense.