I lay my blocks out together and if I like them all next to each other, then I don't add any sashing.
If for whatever reason I don't like them right next to each other, then I lay them out on various pieces of fabric that I think might work and see which I like best.
Then I determine whether or not I want to do cornerstones. And for me the answer is usually yes. Most times I use a contrasting fabric (or sometimes several different ones) but occasionally I will use pieced blocks for cornerstones or add flying geese to the sashing to create stars around the cornerstones.
Have fun playing with the possibilities!