The last quilt I made was a deep burgundy and it wasn't prewashed. A big mistake. When I was assembling the pieces together on the machine my hands were dyed red (from body heat) and the base of my machine has red dye on it. As I was quilting it I was wearing an old pair of canvas shoes and the dye even wore off on those!
This made a pre washer of me! This wasn't "bargain" fabric, either.