If you want to use cornerstones why not make them a different color, one that is in the body of the quilt. It would be a very nice touch and add another demension to the quilt. Keeping the CS the same color is finfe if you want to but not really needed if the same color. You can sew accross a sash to another block without ther being a seam to follow. The choice is yours.
Would love to see a pic!
peace