I generally cut my sashing across the yardage (selvage to selvage) and I also use sashing with cornerstones so I don't have to deal with long strips.

Generally, I sew the sashing strips to the sides of the blocks and create the full rows. Then I sew the sashing strip to the cornerblocks and complete the full rows. I don't press until I have everything done and make sure that I press so the pieces all nest nicely.

Never had a problem using that method.