I've gone to Mexico 8 times in the last 5 years. Don't change any money into pesos. The ports where the cruise liners stop are happy to have your dollars. For a weeks visit to Mexico I usually take at least $50 in singles for tiping, to purachse items from the street vendors and for taxis. All of my other purchase are made with a credit card.
If you need pesos just look for the "Cambio" sign. Dollars don't get great exchange rates from merchants unless you know the going rate and agree to it before purchase is finalized. (There are Cambios on every other corner because it's a big business and they will exchange at a published rate.) Banks will give a slightly higher exchange rate but usually there are lines to wade through.