I prewash everything, and find most will shrink more in one direction than another. I wash my layer cakes, and routinely they go from 10 square to a rectangle that is around 9.75"x9.5". Don't think I've ever had one that did not shrink up unevenly. I wash charm squares too, and they do the same thing.
This does not seem like much but if we expand it to 100" x 100". we'd end up with 97.5" by 95", so a 2.5% shrinkage in one direction and 5% shrinkage in the other.
I also use color catchers and 90% of what I wash (and I have a large stash now) has left die on the catchers. Though I've found it's more an issue if a fabric will accept loose die than if a fabric will lose die.
I had one quilt with a navy that still bled after a few washes. I used in as a background in a scrappy quilt, and three or four lighter fabrics were affected, but more of them stayed white/pink. It was very interesting to see this.