It it were mine [and I am very new to this] I would start in the middle with a walking foot and SITD along each sashing side and then probably do a diagonal line both directions through the big squares. It would very much depend on your batting choice though. Some need closer stitching than others. Warm&Natural I think only needs to be quilted every ten inches but some need every 3 inches.
I would probably use my embroidery machine to do a design in the center of the big blocks, too. Maybe something circular on top of the x as stated above.