Here's my method for lining up sashing strips, assuming the blocks are somewhat uniform.
Sew the first row together. Sew the long line of sashing underneath that row. Turn it over. Using a ruler and pencil, extend each block's seam lines by *marking* that long line of sashing. Basically these are virtual seam lines in the long sashing strip.
Sew the next row together. When sewing this row to the long sashing strip, first match each seam line to each sashing mark and pin. When you sew this row on to the sashing strip, ease or stretch as necessary to keep those pins lined up.
Believe me, this method works and it's easy to do.