I actually disagree: there is a right answer, it needs to be below when rappeling. In the climbing world, autoblock above is NOT best practice for rappelling. As stated above, IF the knot is above, then you are weighting the friction knot 100%, not using the belay device as designed(where the knot is just holding the brake strand in the correct braking positing. IF the friction knot failed(or slipped--had a twig get caught in it, sweatshirt string, whatever to keep it from compressing correctly), and you were hands free the atc would create zero friction and you would fall. IF you are using a prussik knot above the ATC to go hands free, the ATC should be properly tied off using a mule hitch and backup. I"m not saying a autoblock or friction knot should never be above the ATC(often times this is necessary to pass a knot on rappel, of to slingshot into an ascension setup), but that it should not be for rappelling. IF you are having issues with your autoblock running into your device when it is below the device the correct solution is to extend your belay device with a single length sling or such. It actually works better on rappel that way anyway, and gets the device away from any other hanging pieces of clothing, hair, gear, that might get stuck in it on descent.