I do not prewash before using. Unless it is a flannel fabric.
I do only buy better quality fabrics, though I try to find it on sale. Most people in our guild (over 300) do not prewash and non of the 10 or so ladies that I quilt with monthly pre-wash their fabrics, unless it is flannel. I do wash all quilts before they go out the door, after they are finished.
I have never had a problem with dye's running. I do use at least 2 dye catcher sheets when washing.