Originally Posted by Prism99

Guess I am the odd one out here. I probably wouldn't pre-shrink it. This is because my quilting lines tend to be pretty close together (usually not more than 2" or 3" in any direction). This type of quilting binds both top and backing fabrics to the batting, at which point the batting limits how much the fabrics can shrink. Fabric that shrinks a lot when washed alone will shrink very little when washed after having been quilted with lines 2" apart.
If I were planning on quilting lines being 5" apart, I might prewash the muslin. This is because over a 5" span, it would be possible for a highly-shrinking fabric to shrink a little more than the batting. However, before washing that large piece of yardage, I would probably do a test wash first on part of it. I would cut a 10" square from the muslin, wash and dry it, then measure the ironed piece. If it shrank an inch, I would probably prewash the whole piece.

A lot depends on how far apart your quilting lines will be. I enjoy quilting on my midarm frame so, instead of prewashing the musline, I would just plan on doing my usual close quilting.