I don't generally do sashing as I prefer my blocks to make secondary designs where they join.
Sashing is a quilter's preference, though generally based on the size of your blocks.
Can't you just strip cut the sashing pieces? The corner stones are usually square and may be small block themselves or plain squares as your preference warrants. For simple blocks I'd use a plain corner stone. You don't want the sashing to overpower the blocks; just set them off.