I'm a big believer in prewashing and for that reason I don't typically buy precuts like jellyrolls. Long skinny pieces knot up the most during prewashing, but small squares/scraps can be washed in lingerie bags which helps a lot. On a side note, after dealing with scraps and stash for decades, in the last couple of years I've decided that I simply don't want to store/deal with pieces less than 6.5 x some sort of rectangle. Even if I'm cutting 2.5 strips, I can always cut down my larger pieces but I can't cut "up" a jellyroll.
I do color test specific pieces of fabric, but usually wash like colors along with my regular laundry. I do throw in a dye magnet thing, if it comes out darker than expected I re-examine the colors. I really dislike the ironing part and find about 6 yards of fabric is all I'm really willing to deal with in a single load, and if possible I like pieces 3 yards and less. Fabric preparation day is part of the process but is not my favorite step.
The prewashing does clean the fabric -- I've seen some packaged fat quarters particularly that had grimy edges. It removes any scents, sizing, starch etc and lets me know the true condition of the fabric. I am "sensitive" to many chemical smells, they make my nose run without technically being an allergic reaction.
The washing and drying sets the shrinkage rate, and if the fabric doesn't hold up to use, well I know it sooner rather than later. I've had expensive "brand" name fabrics turn flimsy as cheesecloth after washing, and then I've had cheap ones that never softened up! Both fail. But once my fabrics have been washed and put in the stash, they are ready to go.