I had a 108" wide backing shrink SIXTEEN INCHES in width after I washed it!! Sure glad I prewashed/preshrunk it.
No kidding -- it started out 108" wide and came out 92" wide! Good thing I'd bought extra length as I had to cut the extra length off, cut that in half, and sew the pieces onto the sides of the backing to get my width. Painful experience.
I don't like all the endless ironing after having washed & dried but ... just imagine how horrible it'd have been to have put in eons of work on my quilt (Tumbling Blocks) only to have it shrink like that!
You cannot depend on who may eventually end up with the quilt to not wash & dry in hot water so I make sure of the shrinkage and fading FIRST.