On my antique Duramax/Allison truck, I use TH any time I'm towing. Slightly higher rpm shifts help keep it in a good torque range and keep EGTs down on hard acceleration, like merging onto 70 mph freeways. Keeping it in TH helps in heavy traffic-the extra braking comes in handy when idiots dive in front of you and stand on their brakes to make that offramp and, of course, the extra braking on mountain roads is priceless. Use TH when towing, that's why they designed it and called it "Tow/Haul".....
2004.5 GMC SLT CCLB, Ride Rites, Turbo brake, EFI Live
2012 Arctic Fox 29 5T 5th Airborne, B&W, Prodigy
Papillon navigators Lily, Buddy and Willow