Try it both ways and adjust for driving conditions is my advice. I don't use it going down a flat interstate on the Plains. I have had my rig a couple of years now and have had trouble getting use to the fact that these Fords are just different. They seem, as many have commented, to love high RPM. I guess I am old school, because I always tried to keep the rpms down. Tow/haul is really great for long steep hills, up or down.
OK, I confess: on mountain passes I prefer to shift it myself and have a little more control, watching the tach, speedometer and "feeling" up the mountain with my foot on that accelerator. I am just more comfortable that way, "flying by wire" instead of the computer.
Paul