it is a personal choice...some people pre-wash to prevent shrinkage in their finished project, some pre-wash to remove sizing/chemicals in the fabric used during the manufacturing, some pre-wash to make sure no fabrics will run. some pre-wash because that is what they were taught to do...some never pre-wash anything- and take their chances.
me- i prewash fabrics that...feel funky- may bleed- or may shrink more than others being used-or if their is an odor in the fabric.