Here's how I do it. I sew the rows of blocks with short sashing strips in-between. I add a long sashing strip to the first row. Before adding on the second row, I turn that first row over and -- using a pencil and ruler -- mark all the seams lines from that first row onto the long sashing strip. Next step is to *pin* the second row to that long sashing strip, matching its seams with the pencil marks. When I sew, I then ease or stretch as necessary to get those pins matched.
You can do even narrow sashing this way and have all the blocks come out even!