I ALWAYS prewash and have had the unfortunate luck to have many fabrics run. It isn't always reds as others have also mentioned and I have had fabrics from JoAnns run as well as quilt shops. I've had issues with blues and blacks as well. The latest JoAnn fabric was red with little white triangle trees and it was very inexpensive. I have an asthmatic in the house and am very warry of excess chemicals in our home environment, the only reason I haven't invested in color catchers.
Prewashing also makes it easier to grab my fabric and go, change my fabric choices at the last minute or whip up a project on the spur of the moment and they all play nice together. Most of my projects would be washed frequently anyway...I rarely do projects other than pin cushions and ornaments that aren't washed (and washed often!!) :D