I had not thought of marking where the seams should meet on the sashing. I usually just sew the sashing on one side and then using my ruler and pins mark with a pin on the sashing where the next seam should be and then sew it up. It's mostly good, but every once in a while I get off. Marking where all the seams should fall on the sashing should give you a much better end result. I'd check any marker I used and make sure it will go away after you get through. I probably would have to mark it with the lightest color I could find of my choice of marking media, on the back and match it up that away. You could, also, mark in the seam allowance and just a couple of dots on the middle of the sashing.