It really does not matter as you are only trying to prevent the cool air from pooling on the bottom and the warmer air from pooling near the top. This helps the air movement when refrigerated items interrupt the normal convection currents produced by the evaporator. The reason the evaporator is placed high in the reefer is that the cold air it produces naturally sinks to the bottom and the warmer air will rise and be cooled to continue the process. With items in the reefer, the perfect convection currents are interrupted and allow for uneven cooling of the food.