The suggestions about cutting are great -- starching too. The other thing to watch out for if you cut a lot of strips is the bow in the strip.
However, at this point, I would draw a lines across the back of the sashing fabric like Prism99 said that are the average width of the stripes, then match the strips to the to the lines and pin. I would probably sew with the stripes on the bottom if there are a lot of them that are wider than the average. The feed dogs will ease in the fabric a bit.