Definitely long pieces of sashing. Sashing each individual strip would (in my opinion) open the door for more wonkiness, with all the extra seams.
You will want to make sure your columns are straight, before you add sashing.And you'll have to take care when joining hte columns together that the coins on each side of the sashing line up in even rows, without getting offset/wonky on the way down the strip. Best option for that is to mark the sashing where each seam should hit, and make sure it stays aligned while stitching.