I have a 2013 TDI 8 speed and I tow a 3500+ TT and I stay in "D" almost the whole time. The only time I force a downshift is climbing long uphill grades when the engine might start dropping in RPM. Otherwise, I stay with the cruise set at 65mph and the Touareg doesn't care. RPM's are about 700 higher than normal (not towing) for any given steady speed, and the transmission temps are under 200F all the time (High temps, 240+F are transmission fluid killers).
Agree. +1
Transmissions tend to overheat when they are doing the work instead of the engine. It is much better to have higher engine revs and say be in a lower gear than the inverse. This was why in older style boxes people stayed out of overdrive.
As others have said, this is a strong box but that doesn't mean you shouldn't look after it. If you're going up a long ascent and the car has put itself in say 6th and it's on the point of changing to 5th but the revs aren't quite low enough for it to change, this is the only time I would shift it manually to 5th.
With respects to the OPs original question, it actually doesn't matter whether or not you are in S or D, the point of good towing practice besides maintaining control, looking after the vehicle and tow package etc is to stay in the torque band.
This is the same when off-roading or doing any heavy "load work" such as sand driving.
Look after your touareg, they're a beautiful thing and they do have a soul and feelings.