I use 2.5 when I'm piecing. Short enough to hold the seam securely. When I SITD, I use a longer stitch length, usually 3.0. I use a really short length, like 1.5 or 2 for loosely woven fabrics, like homespun and flannel. Another time I use a short stitch length is when I paperpiece. Perforates the paper more to make it easy to remove.