I go 60 mph 99% of the time. It's what's recommended by Toyota and only 5mph below the tire rating.
I have gone faster for brief periods while passing in order to free up the passing lane for other traffic, but that's rarely needed.
If I feel the slightest twinge of being unsafe (very high winds, heavy rain, come to mind - I never go fast enough for this to happen in good weather), I slow down. Others can wait. All in all, my towing experience is very smooth and pleasant. I don't like white knuckles, so don't do things to cause them.