For chicken we use the highest rack available, in my opinion it helps with color, texture and skin.
For ribs I am leaning towards middle to lower racks for a lower and slower cook. I find in my party model if I put the ribs up to high, they get dark to quickly.
I also cook with sand in the pan and not water. I have found the food usually has enough moisture, and the cooker is so well insulated, that water is redundant.
Just my .02 cents ymmv.