Not sure if anyone else posted this tip, but it's how I get my sashings perfect when not using cornerstones.
First step is to join the blocks into rows with sashing. Second step is to sew the long sashing strip onto the bottom of a row. Here's the tip. Before joining the next row to the sashing strip, turn the first row with sashing strip to the wrong side and use a pencil and ruler to mark all of the block seams onto the long sashing strip. When joining the next row, pin that row's seams to the marked pencil lines on the sashing, easing wherever necessary. This will keep your blocks aligned from row to row.
Edit: This technique works whether you sew rows first or columns first. Just apply the pencil marks to the long sashing strip.