I prewash - that is what I was taught years ago when I learned to sew. I have had fabric run on each other in the wash, luckily they weren't in the quilt. I have also had problem with shrinking, big problems!!
Current example - do I or don't I wash my jelly roll. It has browns, reds and creams I decided I was gonna wash and them in color batches in a lingerie bag. They shrunk and raveled!! My pre cut 2 1/2 strips are all 2 1/4 inches now. Didn't realize they shrunk until I was sewing the other strips to them that I cut from prewashed fabric. So now I have all these pieces sewn together that I have trip to 2 1/4 augh!!!!!!!!!!!! I could scream infact I have many time and some not nice words have come out of my mouth while working on the quilt. :evil: :evil: :evil: :evil: :evil: :evil:
I wash and iron when I get it, the bag from the fabric store goes straight to the laundry room.