Mildew will not grow on modern synthetic sailcloth, but it can grow on the dirt that it accumulates if moisture is present. Although mildew will not harm the fabric, it will discolor or spot it. Fold the sail and store it in a cool, dry place. The method of folding that you select is not very important, though try to keep folds to a minimum. To that end, let me quote from the literature distributed by Glen Raven Mills, the manufacturer of Sunbrella.
Spot wash by sponging briskly with a soapy solution of natural soap in lukewarm water.
Rinse thoroughly with clean water to remove soap and air dry. For stubborn stains or suntan lotion, use a fabric remover. Follow directions on container. Rinse thoroughly with clean water, and air dry. Solution-dyed acrylic does not promote mildew growth.