Whether or not you can do decorative stitches with your walking foot really depends on the model and brand of your machine and the walking foot that you are using on it.
I heavily starch backings before layering. My method is to mix a 1:1 solution of Sta-Flo liquid laundry starch and water, "paint" this solution onto the backing fabric using a large wall painting brush until the fabric is saturated (my kitchen island is the right height and makes for easy cleanup afterwards), allow a couple of minutes to make sure starch has saturated all the fibers, toss in the dryer, then iron with steam. The starch stabilizes the backing fabric so it is much less likely to stretch or distort while machine quilting.
With an already-layered sandwich, it can still be worthwhile to give both top and backing several layers of spray starch to stabilize the fabrics. You can do this by laying the quilt on a large flat sheet, spraying the starch on (from sides to center minimizes overspray), letting the starch dry (a fan speeds this), and doing it several times.