When you think you have them lined up you can either hand or machine baste a few stitches jsut before and after the sashings on the seam line. Then check to see if they match. If they do then sew the whole seam.
You can also glue the sashings when they meet with Elmer's School Glue.
Any of these ways, you are not ripping out whole seams and when you go to sew, the matches will stay put as you sew over them.