Supply/return plenum box: OK to use top of box?
The reason there are no supply ducts running from the top of the supply air plenum is that the air from the fan in the air handler hits the top of the plenum forcing air equally to the supply runs coming off of the sides of the plenum. If you were to have a supply run coming off of the top of the plenum it would get more air because of it having the path of less resistance. Return air plenums do not matter where the duct openings are as much.