I agree with Peckish. I love working with muslin. If it is quality (e.g., RK) muslin, it really shouldn't shrink noticeably more than other quality cottons (like Kona), assuming you was on cold/gentle cycle, and dry on low heat -- which is how I wash all my quilts.
I am making a Calico Gardens quilt that is appliqued prints on an muslin background. I did not wash the muslin before starting, nor did I wash any of the other fabrics. I have not finished mine yet, but a number of other girls in my class & the teacher all skipped pre-wash (because we received each month's kit at the start of each class) and so far they have all laundered just fine.
If it's a lower quality muslin, I agree with CMW -- denser quilting with a cotton thread will reduce likelihood of shrinkage during laundering process -- just know that typically will slightly reduce the size of your top as a small percentage of fabric is taken up in the stitching.