Where to place an oven thermometer in a gas oven? Should it be left hanging from the rack or should it be placed on the rack?
I tend to just stick it close to where the food cooks. From the two options you name that would mean putting it on the rack, not hanging under it.
I checked a couple of sources and found this one quite a nice read.
I believe that if you have a convection oven that it doesn't matter, heat is distributed evenly throughout. If you have an electric oven than the temperature is most accurate towards the back. You may notice that when the oven is on that one of the back burners will radiate heat. I get the most accurate temperature reading with mine hanging off the top rack in the rear. That way when I open the door the gauge doesn't drop right off either.