It depends upon how much you enjoy gambling. 100% cotton fabric usually shrinks, at least a little (sometimes a lot). The color (any color) may or may not bleed. Depending upon whether or not it was wound on the bolt perfectly straight (before it was cut), it may or may not be the same shape, after washing. Some people swear they've never had a single problem, not pre-washing fabrics. Others have had quilts ruined, as a result of not pre-washing fabrics.
I always pre-wash fabric. I like to know that when a quilt is finished, it can be washed/dried, without worry. I don't like the idea of having to use color catchers . . . or worse, telling whomever I gift with the quilt, that they must use color catchers! I've had quite a few fabrics bleed (quality fabric, not cheap stuff) and a few never quit bleeding, even after multiple washings! It does take a bit more time and effort, to pre-wash . . . but it's a lot safer, IMO.