recently a Quilt Store owner said she doesn't prewash unless she is concerned about bleeding, that fabrics are pre-shrunk.
I still pre wash. And I think I will continue as I have seen some shrink even recently.
I'd hate to spend my time on a quilt and then have a patch tear because that fabric decided to shrink.
With kits that are already cut though I don't think you can prewash as the edges fray and then you won't have the right size.