The only plane I had that required down elevator was a plane that had the rudder hinge line angled slightly back as opposed to straight up and down. Ever notice that some planes have that hinge line angled back? Whether they do this for streamlined looks or for any purpose I am not sure. If the backward angle is too much it will pitch the nose upward much like a v-tail plane will use "up elevator".

I think that with most planes, the yawing action from the rudder causes the nose to drop and some up elevator is needed.
Ernie