As is known for Shimano sealed bottom bracket cartages (e.g. page 16 here), if the BB cup is made out of metal, grease should be applied to the frame-cup thread. If the cup is plastic, grease should not be applied.
My question is what is the purpose of applying grease?
On a side note, I have heard of a couple of explanations, none of which I can prove or disprove. Firstly, to prevent difficult removal of the cups due to corrosion of cold welding. And secondly, to prevent intrusion of water and contaminants through the threads.