All cotton fibers shrink with water, hot or cold. If they are made into thread, it will shrink when subjected to water. If it is woven into fabric it will shrink. The warp and weft of fabric will also shrink. The threads used are not generally balanced in both directions. But, cotton fibers made into quilt batting also shrink. I do not pre-wash because I know that all components will shrink when the quilt is washed. The amount of shrinkage is more controlled (at least I think so) when everything is washed at the same time. It depends on the look you are happy with.

I wash all the fabric when it comes in the house. Last week I purchased black and red fabric to make sashing around two quilts with great blocks. I got the fabric at JoAnns and from the get go (from all the information from this wonderful Quilting Board) get it to the washer. I started with the red first. I always wash in warm water and threw in 5 color catchers and the catchers were all bright red. I rinsed it again with 5 more catchers and more red. Finally, threw it in dryer for 15 minutes and hung it up. Same thing with the black fabric and the same with the dryer. Now, when I give anyone a quilt anymore, I tell them to wash it in cold or warm water and use color catchers. That is the best I can do.

I agree with your observations, and it is also why I usually prewash as I don't prefer the 'crinkly' look after a quilt is washed. I can verify that steam pressing already cut out pieces Do Shrink. I had this happen to me a couple of years ago, when I did Not pre-wash, and had cut several 8" squares for a quilt. They had gotten wrinkled in storage, so I pressed them (with steam as I always use steam), and when I went to use them I found they had shrunk down to 7 1/2"-7 3/4" and they would not fit in my quilt assembly. Learned that the hard way.

And flannel? Boy, that stuff can really shrink....

i actively dislike " the crinkly look".

my quilts get "some puff" anyway - even after using washed fabrics and batting.

I dislike the crinkly look, too. I had a RJR fabric shrink when pressed. I only needed a few strips, so I didn't prewash that one. it was a strip quilted block and when that one fabric shrunk, it distorted the block. I had to replace every peach print strip - after washing it, of course.
I prewash most of what comes into the house. A few that don't seem to be offgassing are stored unwashed, but will be washed before cutting. Now I'm starting to itch thinking that I may have bugs or whatever in my totes.

I also pre-wash as needed. A lot of my fabrics are made into wearable art garments which will never be washed and I like having a bit of the original sizing in them when working with them so I only pre-wash when I am going to use them in a quilt. Even then it's a bit spotty. I love crinkly quilts so don't worry that much. So far, so good!

Thanks for reporting on your experiment. It confirms what I've always thought, although I prewash less these days. I always prewash batiks and red family colors, but not other fabrics. I find I can cut unwashed fabrics more accurately without washing, and I do like the "puffy" look. After all, when you use a quilt for a while, it will need to be washed and then will get puffy anyway.

I pre-wash when I use the fabric, unless it is a pre-cut, then I wash before gifting with Synthrapol and Color Catchers.
I quit using steam a year or so ago when I saw it distorted or shrunk the fabric.

If you don't pre-shrink your batting and it is something other than polyester, won't you get the crinkly look anyway?
Personally I don't mind the crinkly look as it makes my comfort quilts appear vintage.

I always pre-wash.