I have the same question. I hate using the T/H mode personally. I'd rather use the gear selector with the the truck in 5-/5+ because it seems to shift so much better that way. I mean it's a very noticeable difference to me at least.
I watched the gauges while towing on the grades under both conditions and didn't notice any big differences between T/H and not. The hottest I saw the oil get was 246°, the trans was 176°, and the coolant was 222° and then the coolers would kick in. This was towing around 10.5K. My normal non-towing numbers are around 206° oil, 168° trans, and 204° coolant.
However, if its truly better to use T/H mode I'll use it.