2Hams,
Your point about the extra mattress (and pin) weight is right on. It reduces the "tail wagging the dog" affect from too heavy of a tail versus pin. If your setup travels well on (1) down hill curves and (2) mild zigging and zagging between lanes, don't worry about it. If either is white knuckle experience, moving a little weight forward will help, sometimes a lot.
Within reason, I think it would be safer to slightly overload the tow vehicle than have too heavy of a tail that can toss a tow vehicle around pretty easily in an emergency situation.