The short answer is, it depends. I've done decals before and after chipping. If you're going to do them before the chipping you'll need to protect them from the hairspray and the rubbing. A couple coats of a clear varnish ought to do the trick. So here's how I would do it: paint, clear coat, decals, clear coat, hairspray (or other chipping medium), whitewash, then a top dull coat.
For your first time, I might suggest doing the decals after the chipping; but you do you!