The choice of whether or not to pre-wash your fabrics is up to you. Some fabrics may bleed colors or shrink, so this may be a reason to pre-wash them. On the other hand, you may find crisp, unwashed fabric easier to work with.

Pre-Wash Fabric
Unfold fabric to a single layer
Wash in warm water with a mild soap
Rinse the fabric until the water is clear
Dry in the dryer (along with a terrycloth towel) until slightly damp, or hang to dry
Press the fabric
If fabric crispness is desired, spray with fabric sizing (like starch, but lighter) before pressing

Test Dark Fabric for Bleeding
Cut a small square of fabric and place it in hot water in a white cup
If the water turns a color, soak your fabric in a half-and-half solution of white vinegar and water
Dry and retest
If it still runs, repeat the solution process
If the fabric continues to run, do not use the fabric in a quilt