The denser your quilting, the more it "draws in", or pulls together. If you are planning a small stipple in your blocks and only STID for the sashings, the sashings are likely to look puckery, even if you anchor them with STID first. That small stipple is denser quilting. Try doing some sort of squiggle or loop in the sashings to add more quilting. It will absorb any puckering and look more balanced. Or you could quilt diamonds, lines, anything to add quilting in the sashings.