There is no need to take maleria tablets if you are just visiting Cusco and Machu Picchu. Maleria is not present here at these altutiudes. However, if you are travelling to the Amazon Jungle, you may wish to seek professional medical advice.
Just a note about Machu Picchu! Although you shouldn't be concerned about maleria there, it is advisable to wear long clothing and closed toe shoes. There are some nasty little sand fly's there that like nothing more than to bite tourists. The bites are nasty and itch for several days after your visit. Be prepared with insect repellent to spray on your skin and clothing. A deet based repellent is recommended.