I have had tremendous success with a method that I learned in Weeks Ringle and Bill Kerr's book, Quilts Made Modern. I will try to explain it. After you have your coins stacks assembled (which it appears you do), don't cut long narrow sashings. Instead use a strip of sashing fabric that is a few inches longer than the length of your stacks, but maybe 20 inches or more wide. (I've even used the whole WOF.) Sew your coin stack on to that wide strip, press your seam open and then cut the wide strip down to the width you want your sashing to be, measuring from the seam and adding 1/4" for your seam allowance. Sash every coin stack in this manner, continuing to use that wide strip (which will keep getting narrower.) When all your stacks are sashed, sew them together. Be sure that if you want the outer borders wider than the interior sashings, you cut them wider. I wouldn't worry about trying to match seams horizontally across the quilt. Just be sure that they are even across the top and bottom. Hope you can understand this. I've made three quilts in this way, and they were perfectly rectangular - no wonkiness at all. Good luck. I love your fabrics.