Keep in mind that most backing will shrink quite a bit if you prewash. I've not ever tired using it without prewashing and then measuring how much the finished quilt shrunk up.
When I made a quilt for my king size bed, I wanted it to end up close to 120x120 when quilted and washed. I buy 108" wide backing and it usually shrinks up to around 102"-104", and then I trim off the selvages so end up with a bit less.
I have bought 120" wide "preshrunk" muslin, but it also shrunk up a bit in the wash, not near as much as regular quilt back fabric though.