Cornerstones help line up sashing, and I use them when they are part of the pattern. However, more often I just want plain sashing. In that case, a lot of extra cutting and sewing is eliminated by using long sashing strips to separate rows. Here's what I do.
(1) Create rows by sewing short sashing strips between the blocks in each row.
(2) Before attaching row 1 to row 2, turn row 1 to the wrong side. Using a ruler and pencil, line up the ruler with a seam and use the pencil to mark the seam on the sashing. What you are doing is creating "virtual" seam lines that do the same thing cornerstones do.
(3) Pin row 2 to row 1, matching real seamlines to pencil marks.
(4) Sew, easing or stretching slightly as needed.
(5) Use ruler and pencil to mark bottom of row 2 just as you did with row 1.
The pencil marks ensure that the blocks are all lined up when you are finished.