Magnetic drain plugs are always a good idea, as any metal that is floating around in the oil (unless it aluminum) should be captured by the magnets. You are right that there will be more floating around during break in, but general wear will cause fine particles to always be present. I'd rather catch as much as possible, in as many places as possible, to keep from wearing the internals any more than I have too. I have a magnetic drain plug, and a subsequent rare earth magnet on the outside of my oil pan on my 92 Toyota Celica, and it just turned 290,000 miles today, with no work other than one head gasket ever having to be done on the block. My oil is always golden in color at each oil change (every 3,000, or less). I know that I always have a little fine particles on the plug, and a little near the other magnet. Better there than thru the engine !!