If you want narrow sashing between the blocks you can slash the blocks and put it in but you still might get some wobbles. You could try appliquing strips on top or maybe even do your quilt QAYG for the narrow sashing. QAYG makes it easier to quilt blocks and then trim square before adding sashing. It is a little more work to then put sashing on the back but this might be a method for perfectly straight narrow sashing.