@STARKMAN
Not exactly. A recipe will instruct you to put a large roast on a bottom rack because it wouldn't fit on an upper rack due to it's largeness. Temperature is controlled by that big knob on the front of the oven.
Having said that, there is a temperature differential between top and bottom of the oven, as much as 25F. Your oven may vary, depending on size, age and presence of a convection fan. Use two thermometers and test it for yourself in your oven. (2 thermometers are required to get accurate simultaneous measurements of top and bottom.)
So if you are cooking two items (breasts and thighs or a roast and vegetables) in separate pans place the one you want cooked more gently on the bottom. If it is all in one pan then it doesn't matter, use the knob to control temp.
Another example would be panko coated chicken breasts on the middle or lower shelf and a pan of steak fries on the top shelf. The higher heat at the top encourages crisping and browning of the fries and the lower temp on the lower shelf helps keep the chicken moist.
It is a good idea to know your oven though. When you set it to 350F what do you really get? Is it 330-bottom, 350-middle & 370-top or is it 325-bottom and 350-top? The only way to know is to test it. Also, what is the cycle rate of your oven. The temp fluctuates around the average temp (which is supposed to be what you set on the knob, but often isn't) and the better ovens maintain a closer temp to the goal temp and cycle in a shorter time. This is a bit harder to test without a data logger but can be done by taking readings every 60 secs for 30 mins.