I always pre-wash and all my fabrics no matter how small. My reasons, 1.If it is going to shrink it will before I use it, when I piece and quilt it, there won't be the tension on the threads pulling my stitching apart that would be if it isn't prewashed and shrinks in the finished project. 2. when I prewash I toss in the shout color catcher, and whatever dyes are released they won't be ruining my finished project.
I wash fabrics before putting them in with my stash. Sometimes with towels, or sheets, other times with regular laundry, and sometimes if I have a lot to wash I'll do a load of just the fabrics. I often will use a 2nd rinse too.
If I am spending my time to make a quilt or wall hanging I'd prefer to give it a fair chance to last as long as possible.