I would. This is one reason I'm not buying so many precuts anymore. I want the option to wash if I have a bleed-y color.
I read where someone took a small square of each of the colors she was planning to use and sewed them applique style to a piece of white and washed it to see which ones ran. I think she ended up making the piece into a pillow after she was done using it for testing.
Also I'd use a color catcher even if you prewash. That way the dye doesn't come off and then redeposit onto the fabric (only to come off again the next wash).