My machine defaults to a 2. something. It seems to be just peachy for normal piecing. If I am paper piecing I drop it down to a 2.0 or less. If machine quilting (rarely) and/or attaching binding, I will up it to 2.6 or better. Generally speaking - more layers - longer stitch length. Paper piecing, you can stay with the default but it makes it more difficult to rip out the paper and, during that process, has a tendency to loosen the stitches, hence the shorter stitch length for that technique.