Sewing Tip: If you chose to use a turned-and-stitched seam allowance finish, it is recommended that you apply a zigzag stitch or serge the individual fabric edges before folding and topstitching them as shown below.
Note: This step does not apply to no-fray fabrics like leather, suede, felt and multiple types of knits. You should use overedge stitch particularly with medium to high fraying fabrics.
While a folded and stitched finish provides a cleaner way to finish seam allowance layers, if not prevented from fraying, the fabric edge swill continue to fray (especially after machine washing) even if they are not visible on the wrong side of the garment. To add an extra layer of functionality to this seam finishing technique, you should first contain the edges with an overedge stitching application like serging or zigzag stitch. These stitches will not be visible on the applications right side since the edges are folded inwards, thus they won't effect the finished seam allowance edges visually. Nevertheless, the addition of this step will contribute to the garment longevity, durability and of course, quality.