I pre-washed 5.25 yards of backing for my current project. The edges were serged first and then trimmed so that was about an inch. Trimming off the serged ends lost another inch. Measured again and it was 4 inches shorter so there was some shrinkage.
A friend says her king sized quilt shrunk about 8 inches after washing. She doesn't pre-wash fabrics.
I would rather account for shrinkage before cutting the fabric and quilting so I will pre-wash. Plus I don't want to pay for quilting that is going to shrink away.