Had this discussion with a lady this morning when she asked when I used mine! I told here when starting from a stop loaded or trying to start up on a hill, even now with a motorhome. She said only to use for downhill according to a website she visits. I do sometime but not often as you can well control your speed unless on a long descent. Same on long climbs if you have the speed to carry you through.
T/H locks your trans and keeps the converter from shifting automatically. So that is why it is not needed on flat terrain or for all pulling duties unless you keep the truck/vehicle bogged down in slow speeds and low rpms. Have 04 F350 and now E450 and t/h seldom used on both. Read your manual, look for references but if you use t/h all time I believe you are wasting the fuel you are trying to save in the 1st place! Just my thoughts and yours may vary.