This is an easier method rather than long sashing strips.
http://www.valeriecustomquilting.com...ngSashing.html
I've only made one quilt using long strips with cornerstones across the whole quilt. Lots of re-seaming in that one to make them fit. LOL
Yes, as Iceblossom mentioned, pin about every 2 inches. I have learned to avoid those long seams if possible. Another option is to cut the sashing slightly larger and then square up to finished size.