I have done something similar, making sails for a windmill.
http://i132.photobucket.com/albums/q11/Boney_Moroney/Sail.jpgThe trick is to create the mesh with lines following the creases. Careful selection of nodes, pulling some up and others down, creases the mesh. Finally, convert to subdivided. In my example above I did not perform edge reinforcement, where an extra edge is created just within the edge of the sail, to reduce the effect of subdivision rounding the corners off.
However, In your case, I would strongly recommend creating the sails as smooth regular meshes and applying photographic textures, which include the corner creases. Applying a bump map of the creases should also work well. The creases are such a shallow undulation in the fabric that this should work quite nicely.
I would suggest that it is more important, in sails such as these, to get the sewing seams right, making the sails pillowy in a grid shape. This would be a more pronounced deviation from a flat regular plane than the corner wrinkles, and should be relatively easy to shape in a grid meshed sail, though it may work ok as a bump map as well.