Although it is tempting to SITD starting at a corner, it is still better to start in the middle and stitch out towards the edges. This takes any extra fabric out to the sides rather then trap it in the middle. I would start in the middle of quilt and stitch to a corner, I would then go back to the center point and continue that line out to the other corner. This will give one long diagonal line done. I would flip the quilt over and see if the backing travelled at all. If all looks good, I would continue on. Once all the long lines were done, I would check to make sure the backing is still good. Then I would quilt the shorter side lines being very careful as the sewing lines cross the long lengthwise lines. Those are the spots where if the fabric on the back has travelled, you are going to get pleats. Once all the straight lines are quilted straight, you can quilt whatever you want in the squares.
Or you can forget about keeping the sashing straight and just start working in the middle and quilt your way out to the edges. Completely up to you.
Last edited by Tartan; 01-22-2016 at 11:14 AM.