You don't *have* to wash out basting spray when you are finished. It's just generally done on bed quilts so there are no unnecessary chemicals in the quilt people sleep under. I have used several different brands of basting spray, and highly recommend spending the extra money and using 505 brand. It doesn't stink and just plain works better than the others.
It helps to use good technique when applying basting spray. You don't want to spray the entire backing, then try to lay the batting on top! Google basting spray to find descriptions of the proper technique.