I towed primarily in auto mode, but used manual mode fairly frequently. You want to avoid having the transmission hunt for different gears, that causes additional wear. Equally important, however, is to keep the transmission torque converter in lock up mode on the highway as the torque converter produces significant heat. A long time poster named withidl has summarized this very well over the past few years. Some of his key posts are linked below, have a look:

Xoutpost.com - View Single Post - BMW Towing ?

http://www.xoutpost.com/bmw-sav-foru...-mountain.html

http://www.xoutpost.com/bmw-sav-foru...airstream.html

Xoutpost.com - View Single Post - X5 3.0 Steptronic Question

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue