I wash mine before it goes into the stash or gets used. I had an early (1980s) unfortunate episode with a burgundy running on me after it had been washed, It just takes one bad experience to make you learn. Even with today's fabrics, you find horror stories posted here at least once a month. or two.
Lately, I've been working in orange and noticed the wash water had some dye in it. You can't always tell until it's too late. If you wash a quilt that was not made of pre-washed fabric, you risk puckering if the fabrics shrink at different rates.