I think it doesn't make a lot of difference on how you cut the strips except that you don't want them on the bias...and yes, the edge of the piano key border does stretch because of all the seams, so a basting stitch around the four sides would help keep them stable until you get around to the next border or the binding. also - it's probably a good idea after putting the basting stitches in to measure the opposing sides to make sure they're equal in length...if not, the quilt won't be square...and you could adjust the long side on the stitch line to make it a little shorter if you need to.