I also use the 2.5 default for piecing. Paper-piecers like to use a shorter stitch than this because it perforates the paper more, making it easier to remove. When quilting on my machine I usually lengthen the stitch to at least 3 because of the thickness of the quilt sandwich; makes a better-looking quilting stitch. People who piece and have a problem with stitches coming loose at cut edges often find that shortening the stitch length makes their piecing more secure.
I believe the 2.5 is a metric measurement (millimeters). Vintage machines such as my old Singer use a different type of scale measurement.