Is a currency rate of 0.7789 thru AAA travel decent or should I wait and purchase peso at an ATM in MX?

Leaving for Mexico in one week. Where is the best place in the US to buy peso - bank, AAA?? or wait and use the ATM there?

The exchange rate doesnt fluctuate much... I'd use a bank here in the States, because the ATM will charge you a currency conversion charge, and those charges are usually pretty darn steep ...