I agree with many so far who have posed the question of how you want to quilt this. I'll add by asking if your seams will butt appropriately or are they all headed in the same direction. If you can't rotate the blocks so that all seams butt, then I'd add the sashing. Your quilt will look great either way, just depends on a few things and what look you're going for. Are you hoping to have your piecing stand out or the quilting?