Personally, I would not wash the sashing or border or backing fabric if you are using 5" squares that have not been washed... I think it would cause a bigger problem when you launder it the first time.
I would sew on the vertical sashing first, so that you can sew the blocks into horizontal rows. Then you can cut sashing strips that run the whole length of the row, instead of individual pieces.
I can't get that link to open.....:(
This one will work
http://ny-image0.etsy.com//il_fullxfull.103085060.jpg