I wash all my fabric when it comes into the house. I don't want to have to guess further down the road if it was washed or not (and be sorry that I guessed wrong).
I sort all fabric into light and dark and throw it in with my regular wash and a Shout Color Catcher sheet (you can buy them in the grocery store near the dryer sheets).
I have never had a problem, but I usually only buy better quality quilt store quality fabric (not Joannes or Walmart). But...that goes out the window if I get a good thrift store bargain - then I'll buy anything!!!