Hi,
I never prewash my fabric, and if it is quilt shop fabric it shouldn't bleed. I have read on here that others have had quilt shop fabric (like Moda, etc) bleed....I haven't, so maybe I am just very lucky!
I would make the quilt and then wash it, I usually do wash my quilts after I get them quilted and the bindings sewn. I figure it was handled a lot and quilted and handled and probably on a floor, etc., and I like the look after washing. It seems to puff a bit and be softer.
Bev