I think it varies from company to company. I remember being told in my new entrant training that the best place for them was in the toilet.
That decision is down to the Captain, to be honest if they are already dead diverting seems pretty pointless. I imagine the usual scenario would be that they are in distress, be it by a heart attack or similar, which would warrant an immediate diversion then they passed away before landing.
As a passenger I certainly wouldn't want to be sat next to a corpse!