I don't prewash fabric unless it looks like it might bleed. Then I take a small piece of the fabric and test it for color-fastness to determine if a wash is necessary. It gives me peace of mind.
I recently bought a large amount of fabric at an estate sale; this woman did prewash all her fabrics. Get this: she zigzagged around every single cut of fabric (mostly fat quarters) before washing. I know it prevents tangled threads, but what a lot of extra work!!!