Take care of money exchange, traveler's checks, and ATM transactions in Cusco. A few hotels and shops outside the city do change dollars for soles, but at a less favorable rate than you can get at a Cusco bank. Urubamba has an ATM on the main highway; Pisac has one on its main plaza, ready to supply you with cash for shopping at its market. Aguas Calientes near Machu Picchu has neither banks nor ATMs. Several shops will change U.S. dollars at less favorable rates than you'd get at a bank back in Cusco. Many even trade other currencies, but the rates are laughable -- one Canadian dollar fetches one nuevo sol. You should bring all the money you'll need from Cusco.
