NZQuilter ... Smart plan to serge the edges before washing. You would have the shrinkage dealt with and know for sure what you are working with. Plus that will help to tighten up the weave some and counter some of the potential fraying as you work with it. I wouldn't change my seam widths, but instead, would prepare the fabric with Best Press, before I started to cut and stitch. It will help to prevent fraying as you work with it. Once you have your top quilted, the ends will be well protected.
If you do decide to alter your seam widths, keep in mind that it could alter the results of your finished blocks/quilts and change the proportions, and thus the measurements as you work through the project.
SewBizGirl ... why not wash it before cutting and sewing?
By serging the ends, she has protected them from a frayed mess!