I heavily starch the backing before spray basting and have never had a problem. I do use cotton batting, which is supposed to spray baste easier than poly battings. Also, I think that some brands of spray baste hold better than others.
Starching the backing fabric helps prevent ripples and pleats while machine quilting; the backing won't stretch out of shape while being manipulated under the presser foot. It's just harder for the fabric to bunch up when it is starched.