I prewash or should say pre-rinse. I put my fabric one at a time in the sink in warm water to see if it bleeds. I just had a blue (quilt shop fabric) that bled like a stuck pig. Then I squeeze the fabric in a towel to get most of the water out and throw all in the dryer(for shrinkage). I don't have the thread tangles this way. I know some quilters don't prewash and that works for them. But for me, if I didn;t pre wash, I know every fabric I had would bleed and shrink. Murphy's Law