I carry dollars and pesos in my wallet. Usually run out of dollars..but always have those pesos. You never know when you'll need some pesos...
I also have Singapore Dollars, Aussie Dollars, and Indonesian Ruphiah, but only a few hundred US dollars worth of each. Usually I don't change currency to USD unless I need it so I have enough to get by when I go back until I get set up. No bigger rip off then showing up and needing to exchange current right away for a cab and a meal until I get settled in.