Everybody has to decide what stitch length is best for them and the machine they are using. Practice on a scrap sandwich (even if it just made out of plain white cloth and batting). Make sure to write down the settings that show on your machine onto your little practice piece. Sometimes you want longer, and sometimes you want shorter stitches.
For me, I like to machine quilt when my stitch length setting reads either 2.6 or 2.8. But that wouldn't mean anything to the folks who don't have the same settings on their machines.