There is always a controversy about quilting methods. Everyone seems to find their own way and preferences, such as machine quilting vs. hand quilting vs. tying; basting with thread or basting with pins; building stash or shopping from your closet; cotton vs. poly batting vs. wool or vs. hemp.
Pre-washing helps with the initial shrinking of cotton, which is about 3%. Now this amount seems little, however, if your quilt measures 100" long, that means it will shrink up 3" after it is washed. This is where the puckered look comes from that is characteristic on quilts.
Used to be I pre-washed the batting in my bathtub. However, its a backbreaking task. It also shrinks 3% so it will pull on both the top and bottom of the quilts to pucker and soften.
I pre-wash in hopes that the loose dyes will wash away. It makes the fabric softer on my hands as I work with it. Some of my quilts get washed before I send them, to ensure the colors are set more, especially because I do a lot of scrap quilting. Reds, blues and purples are notorious for bleeding. I have come to love Color Catchers, and even though I use them for my pre-washing, I also include a few sheets with a note whenever I give a quilt to someone.
Also, I laid out one of the quilts to see if I had enough batting and am able to join the two largest pieces I had leftover for a good fit and 3" overlay. This marks a success for the year.
I did not pre-wash the fabrics for the wallhangings. I doubt that they will get machine washed and am also not worried because of the colors they are.
My washing machine was going all day with this and regular laundry.