I used to insist people should go no higher than 4th in the 6 speed or 6th in the 8 speed. That 1:1 (non-overdrive) gear tends to be the sweet spot for gas mileage and A/T temperature control. I still think that's generally true if you have stock gearing and a big trailer. But after re-gearing my 2013 to 4.88's this year, I think I can fine-tune the advice, so here goes...
You can tow in any gear you want, SO LONG AS YOU CAN MAINTAIN TORQUE CONVERTER LOCKUP IN THAT GEAR. That part is key. If you can't keep the TC locked, you're going to generate a tremendous amount of heat. In that case you need to run one gear lower. Now that said even if you can keep the TC locked in your top gear, you probably shouldn't run in that gear unless you have a very light trailer as A/T fluid pressure is lower and won't cool as well. But, so long as the TC stays locked the difference will be small.
If your TC unlocks, or if your transmission downshifts, you need to manually shift to a lower gear - EVEN IF YOU'RE JUST MANUALLY SELECTING THE GEAR IT DOWNSHIFTED TO. That part is also key. And it's a bit unintuitive for most of us. We tend to think "If the transmission downshifted from 6th to 5th then why downshift because the transmission already knows the right gear?" But that's not how it works. With the 2008-2015 6-speed A/T, if you're in 6th gear and the TC is locked, then you approach a hill the T/C unlocks, you speed continues to drop, and the transmission drops into 5th gear. However it does NOT lock the TC while you're in 5th, if you have 6th gear selected. Instead it hums along all day in 5th gear with the TC unlocked, getting worse gas mileage than if you'd just selected 5th gear from the get-go (which would allow the transmission to lock-up the TC in 5th gear).
The thing about Toyota's TC lockup algorithm is that it prioritizes fuel economy over A/T wear (temperature control, specifically). The TC lock is about a 10% fuel improvement, give or take. So if you're going 70 mph and getting a phenomenal 7.5-8 MPG, unlocking will drop that to ~7 MPG. If you downshift another gear you might drop further to 5.5-6 MPG. So, in this CAFE-focused country, you can see why Toyota would opt for economy. If the transmission detects that you need more help to maintain speed (but not so much you're mashing the skinny pedal and forcing the A/T to drop one or two gears down), it will unlock the torque converter first. If that doesn't give you the power you need to maintain (or gain) speed, then it will drop another gear. In lower gears it won't lock the TC because it's goal is to get you back to the higher gear as quickly as possible and if the TC locks up in a lower gear it then needs to unlock to shift.
Previously in my rig (34s, poor aero, stock 3.90 gears) I could keep the TC locked in 4th gear, but 5th was sketchy. If it was really flat and I was going maybe 60mph it would stay locked, but higher speeds or any hills and it would unlock. While locked it would run in the 196-201F range all day. When unlocked it would climb to ~220-225F in the span of a minute or two.
Upon re-gearing I've found I can typically hold 5th gear if it's reasonably flat or long rolling hills, even up to 75 mph. So long as I can hold 5th, my temps are stable (after regear my 4th and 5th gear temp are in the 208-212F range typically. If the TC unlocks though they will rise by +20F pretty quickly. If it unlocks, I manually shift to 4th until I'm sure that I can go back into 5th and lock the TC.
Side note about 6th - I've tried to run in 6th with my 4.88's and if it's REALLY flat and I'm not pushing my speed to more than 60 or maybe 65 I can do it. However the transmission fluid pressure (and thus cooling ability) is directly related to engine RPM. And in 6th gear the RPMs are low enough that fluid circulates slowly and in my experience will run the transmission several degrees warmer (around 217-218F for me, or about +5F to +7F) even when the TC is locked. And don't even bother with 6th gear on stock gearing unless your trailer is so light and small that it's unnoticed (like a small turtleback or popup).