Pretty much never! If I have an engine pulled I may use some alcohol and a paint brush to remove some of the dirt or grit off of them? I fly off of a dry lake bed so there is usually some dirt on them. That's about it. Why people worry about baked on castor on there mufflers or engine is beyond me, it's just the nature of the beast. When I was rebuilding a lot of engines and had enough old engines to make it worth doing I did crock pot a few, no real reason but some people like nice new looking engines?