Here's how I do sashing. I sew the short sashing strips to join blocks together for the first and second rows. Sew a long sashing strip on below the first row. BEFORE sewing on the second row, turn that first row over. Using a ruler and pencil, extend the seam lines across the long sashing strip. This creates "virtual" seam lines in the long sashing strip. Before sewing on the second row, pin each seam of the second row to the matching virtual seam marked in the long sashing strip. When you sew, ease or stretch as necessary to keep those pins lined up.
Blocks are aligned every time with this method, and it is easy to do.