To find out if you need to compound, do a test spot. You want to use the least aggressive product/pad combination to get the results you desire. Then, if needed, switch to a more aggressive pad or polish.
http://www.autogeekonline.net/forum/how-articles/50162-how-do-test-spot.html
Is it necessary? No. Is it advisable? Yes. You don't know how long the car may have been sitting around on a lot uncovered after it was manufactured before being shipped.
Perhaps you may want to test a panel or two to see how much contamination there is to determine if it is worth chemically...
Whereas claying will remove above surface contaminants by shearing, Iron-X will chemically dissolve iron contaminants that are bonded to the surface of the vehicle.
See this article by Mike Phillips on Iron-X...