I always use an airbrush for the exhaust staining. Staining can vary widely, due to fuel quality and flying conditions. I believe that when the throttle was wide open for any length of time it generated a lot of staining.
As far as powder burns, I've never really seen a photo of a Luftwaffe aircraft with significant powder burns, may have to do with the fact that you're moving at a couple hundred miles an hour so it doesn't accumulate. I think some people add it for visual interest but I'm not sure if a historical context exists for it, I could be dead wrong though.
The DB engines were notorious leakers, though. I always like to add some oil staining around the cowl and on the drop tank since you see many photos that exhibit this.
aka Mike, The Mikester My Website
"He has all the virtues I dislike and none of the vices I admire." -Winston Churchill