I don't use cornerstones and have a simple technique for getting sashing to line up perfectly.
(1) Sew the short strips between blocks to form rows. (2) Add a long sashing strip to the bottom of the first row. (3) Before adding the next row, turn that first strip to the wrongside. Use a ruler and pencil to extend the seam lines all the way through that long sashing strip. These pencil lines become your "virtual" seam lines to match up with the next row. (4) Pin the second row to the long sashing strip, matching seam lines with virtual seam lines. When you sew, ease or stretch as necessary to make sure the real and virtual seam lines match. (5) Repeat with subsequent rows.
This is very simple to do, fast, and ensures that your blocks and sashing do not end up wonky!
Edit: The pencil lines replace the seam lines created by cornerstones. Cornerstones are another way to get blocks to line up correctly, but they do change the look of the quilt. When you just want sashing, the pencil method is the way to go.