I am piecing together a lap quilt and washed all the fabric. I was about to use some donated fabric (from my friend) for just a small bit of it (a bird beak or something), and almost forgot to check it. I'm glad I did, it bled like crazy. I just can't imagine doing all that work to have it ruined.
On the other hand, when I made a couple of quilts to hang on the wall I didn't wash anything ahead of time since I will rarely wash them.