I saw a test done by Harriet Hargrave when I took one of her classes. She purposely made a flannel quilt out of unwashed flannel fabrics in her shop, quilted it, bound it, measured it, washed it, and measured again. It was basically the same size as when she started (minus the 2% or so expected shrinkage of the batting). She passed the quilt around in class, so I had a chance to handle it and look at it closely. All of the flannel blocks looked fine. She said she did this to prove to people that fabrics did not have to be washed before quilting, and she chose flannels because they are so notorious for massive shrinking problems. At that time, she used Hobbs 80/20 in all of her quilts. The quilt itself shrank exactly the way Hobbs 80/20 shrinks; just a little softness and crinkling.
After that class, I stopped pre-washing my fabrics. I am not prolific, but I have made several large quilts with un-prewashed fabrics in them that have been laundered in hot water at laundromats. I admit that I machine FMQ more closely than most people; for some reason, I get kind of giddy when FMQing on my Voyager set up and get mesmerized with adding loops and swirls to the quilts.
Anyway, none of the fabrics (some of unknown origin from kits) shrank more than the batting shrank.
I do think that, if using un-prewashed fabric, it's important that the quilting lines not be more than 4" apart or so. Harriet's flannel quilt had FMQ quilting lines not more than 2" apart, as I recall. (This class was many years ago, and my memory is not the world's greatest, so this is my best guess recall.)
I never saw a need to replicate Harriet Hargrave's test quilt by pre-measuring and post-measuring my quilts after laundering. However, it's clear from looking at them that all of the fabrics shrank only as much as the batting shrank.
Edit: Perhaps I should add that Harriet Hargrave's flannel test quilt was lap size. That size would be easier to measure accurately pre-wash and post-wash than the queen sized quilts I have made. That's my excuse, anyway.