While you should backstitch at both beginning and end of a seam, some fabrics can retain some tension from machine stitching. In this case, if the stitch is locked on both ends, there is no way to release this tension thus the final seam will not lay completely smooth resulting in a less-than -professional garment. Luckily, there is a simple technique that will provide a way to fix this tension.
When working with tension prone fabrics, such as lightweight silks and chiffons, leave one of the seam ends not backstitches. You can use this open seam end to release any tension in the seam.
To release stitch tension, iron from the backstitched end to the un-backstitched one until the stitch is smooth and the seam lays perfectly flat. Once the tension is released, you can lock the open stitch end either by stitching in place a few times using the loosed thread and a hand sewing needle (hand backstitching) or by simply tying the sticth loose threads into a double knot to prevent the stitch from puling apart and unraveling.