I worked at JoAnns cutting counter for years---and had a customer come in very upset because she made a quilt--paid to have it quilted...and when she washed it one of the fabrics shrunk. I asked if she pre-washed---and she said "No...you don't need to". She actually threatened to sue JoAnns!! The fabrics all come from different manufacturers...so you never know. I, for one, prewash. But the consensus is probably 50/50.

I would pre-wash that wide back. There have been several posts on this site about the wide backing fabric shrinking excessively.

I pre-wash everything -- big and small, almost all colors. My biggest concern is bleeding, but with large pieces or wide pieces, I have a big concern about shrinkage.