I would suggest laying out some black strips as you would the sashing and audition some squares as cornerstones to see if you like or prefer one effect over the other. You could do a couple of rows and all the same cornerstone, then try those rows with 3 colors, then try an assortment. My first thought was that it is so vibrant and additional small square might compete with the main squares. But as suggested you could also do cornerstones out of the same black as the sashing and have the help of cs and the look of the solid color. I had seen a picture in a magazine of a quilt with squares from stripes and put it on my "want to do it someday" list. Please post a picture when you finish. I'm sure it will be great whichever you choose.