It's not required, however, when you heat them, it makes them easier to fold and less likely to crack when you are rolling them. You can either heat them in the microwave or in the oven. If you are heating them in the microwave, wrap them in damp paper towels and warm them for 30 seconds. If warming them in the oven, heat the oven to 350 and wrap approximately 5 - 6 tortillas in foil and heat them for 15 minutes or so.