Well, depends on what you're cooking.
Go with the middle two grates first. The bottom one is closest to the heat, so tends to cook faster. Heat rises, so the top tends to be a little hotter.
Middle two are right next to the temp probe and tend to be right there.
You need to cook in it and learn the hot spots and use them to your advantage. If I have big butts, I put them on the top rack. If I'm doing a 15lb brisket, with smaller ones, I put the big one in a hot spot to finish.
No absolutes, just put it in, cook and watch the results based upon what rack you use.
The back left corner is also hotter (has to do with airflow)
Smokin'