On hulls I pick up from the ground, I usually give them a wipe with a damp cloth, just to get any dirt, grass etc. off them. The ones I shoot, go from my hull bag back on to the MEC. A little powder burn is usually all that is wrong with them.
I suppose one should give all hulls a wipe off, just to prevent the dies from getting dirty. It might well keep the reloader running smoother longer!