How will the drippings from upper rack meats affect the flavor of the meats underneath? Salmon over the top of chicken breasts wouldn't be appetizing to me.
How accessible are the racks? Which are easiest to get to? Those should have the quickest cooking foods on them.
How even is the temperature from rack to rack? Check the temperature at each level. It's probably a sure thing that it's hotter at one level and cooler at another. Use that info accordingly. Chicken smokes great at higher temperatures, but sausages are best smoked at a lower temperature.
