I PREWASH: batiks, reds, blacks, deep colors, larger pieces of fabrics, anything I am giving away... so almost all of my fabrics.
I DON'T PREWASH: panels, precuts (both rarely purchased)
I don't know where the fabric has been or if it will shrink unevenly or bleed.
Originally I didn't prewash but on the advice of others, I have changed my ways.