Organza will fray badly. You'll want to satin-stitch around the edges of the silhouette to hold it in place. You might try doing this on a "test block" that you can toss in the washer a few times to see if the satin-stitching will hold it, or if it will ravel its way out from under, anyway. You wouldn't have to use the fancy silhouette on the test block, just any shape that has corners to it so you would get a good idea of how it will hold up.