Back stitching is the act of reverse stitching at the beginning or end of a stitch (or anywhere along a machine stitch where extra durability is needed) to lock the stitch threads and prevent the seam from coming undone in the remaining sewing process as well as the finished garment.
While backstitching is essential in the dressmaking process, some seams and vulnerable areas on the garment should not be backstitched as this can create tension and puckering. This is especially true when working with light, flimsy fabrics. Another common instance when backstitching should not be used is at the dart vanishing point.
Darts are conventionally stitched starting at the widest area/intake and moving towards the vanishing point where the stitch blends into fabric surface. While it is completely safe to backstitch at the widest area, especially if sewing single pointed darts, it is best to leave the vanishing point not backstitched.
Reversing a machine stitch at the dart point where the stitch needs to transition as smoothly and seamlessly as possible, can pause severe puckering that is visible on the right side of the finished garment. This puckering is a result of the machine needle catching the dart's folded edges at the vanishing point as it attempts to stitch in reverse.
As noted above, a dart's stitch should end directly on the folded edge at the marked dart point location. To avoid the the potential puckering and tension created by backstitching, leave enough loose thread at the end of the stitch so you can lock the thread with a simple double or triple knot. Knotting the stitch thread at the vanishing point will not only maintain the dart's smooth transition on the garment's face side, it will also securely prevent the stitch from unraveling during the garment's wear and care.