The critical thing is to keep ATF temperatures less than the oxidation temperature for the given ATF. For high performance or towing vehicles, a large external radiator helps maintain a moderate temperature.
All transmissions work best with warm ATF, and some are more sensitive to cold than others. For this reason most vehicles pump the ATF through a tube in the bottom of the engine cooling radiator. It serves two purposes, one to warm up the ATF, the other to cool it. While 130F radiator fluid may hardly seem "cool" to a human, it is cooler than hot ATF in severe service conditions.
A chart (with little to no sourcing information) that gives you some idea how hot ATF runs is here: http://v8sho.com/SHO/ATFTempChart.htm
This link gives some idea how ATF's are tested: http://www.intertek.com/automotive/atf/oxidation/