Tow Haul

I have heard both "all the time" and "when needed." In hilly terrain when there is a lot of shifting it certainly makes sense. Also, it makes sense with a load approaching your payload limits. I do wonder though whether it is necessary on the flat, with a load reasonably below your weight limits. Under such conditions it seems that all tow/haul does is prevent you from going into over drive most of the time. Someday when I have a long flat haul I will check to see if there is any difference in mileage with tow/haul on and off. I look forward to hearing the opinions of others.