I very rarely wash my fabrics unless I know it is OLD and is black, dk green, red or any dark color that may bleed a lot. I buy lower price fabrics from Hancocks, Joanns and a Fabric Outlet and do not have any problems. Bought a dark green at a LQS, tried a small piece in hot water - it bled! Go figure.
I ALWAYS wash flannels because they shrink so much!