I almost always wash unless it is a pre-cut like a jelly roll, charms, or layer cake. I wash fat quarters too and I'm working on a quilt right now where I need to cut two 8 1/2" squares across and some of the washed FQs are barely over 17" after being washed with shrinkage and fraying. I would rather it shrink before I cut out the pieces though.
If you are wondering how much a fabric will shrink, cut a 4" square, wash and dry it, then see what size it is afterwards.