The following is all personal opinion and not meant to lessen your ingenious recycling of magnets
I'm not one to succumb to the whole magnet concept. I understand that there was a time when filtering was non-existent, followed by a time when filters were less than adequate. However, my belief is that that barring catastrophic failure in which the damage is already done, a good filter will trap metal wear fragments to the same micron size as any other non-ferrous particle that potentially could cause the same scouring. And, there is far greater likelihood that non-ferrous particles (i.e., dirt) will be the most prevalent source of oil contamination.
That said, and if one were still in favor of a strategically place magnet, my opinion is that the most probable location to collect metal bits is at the drain plug. This is due to its location at the bottom most portion of any wetted component of the engine and because ferrous metal is inherently heavier than oil, and every time you shut it down, that's were the heavy stuff will settle.
The best defense against the effects of premature wear from suspended contaminants is a quality filter and oil; both changed at or before recommended frequencies
Still, if it provides comfort – use them wherever you feel they work, but IMO – The best place for magnets these days is on the refrigerator.
