Some quilters argue that you do not need to back stitch when you are piecing fabrics together because the intersecting / perpendicular stitch will lock the first stitches in place, thus not causing the stitches to unravel.
I have always believed in back stitching for the fear of the stitching unravelling. Below is an example of the stitching unravelling because I did not back stitch.
I was at a quilt shop the other month, and a lady had brought in her quilt that she had outsourced to be machine quilted. The lady who had machine quilted her quilt had said that a lot of her pieces were coming apart. The end result was the lady had to patch certain areas of her quilt in order for the stitches not to continue to unravel.
If you're going to spend a lot of time and money in making a quilt, the last thing you want is the quilt to fall apart before your eyes.
Quilt & Bitch