[wingnut60]

Terry,
Since I know you have a 450, as I have, I can pass on from experience that you can use T/H or not AT ANY TIME. These trucks/transmissions are very 'smart' and for the most part, will do what needs to be done at the right time.
The 6.4 will tow my 17000+ MS in OD just as well as it will with the T/H engaged, but I don't use it much unless in mountains that require some extra help in braking. Use of the T/H will often delay the final upshift to OD and you will find, as RW and Huck said, running above 2000 rpm at 60 mph (4th/D) when the truck will easily pull in OD to save fuel.
Often, if you are pulling into headwinds, or a grade that hasn't caused the truck to downshift, you will hear the fan clutch kick in to cool the engine that wouldn't have occurred if in T/H--that is one situation that T/H will help over not using it.
Also, be aware that you CAN'T force the TorqueShift into D (4th) with the shift lever--it will always skip 4th and go to 3rd--the only way to access 4th is to let the computer find it for you when needed--so this is another area where T/H may help--it will keep 4th engaged when needed and you can't get to it any other way. This is the only irritating design I have had a problem with in my 450--they should have given us a way to hold 4th manually if needed. As mentioned above, if I encounter a long grade in OD, the truck will gradually lose speed--use of T/H will sense this and downshift; if you try to use the shift lever, it will downshift to 3rd and cause a higher jump in RPMs than necessary.
Another TorqueShift pecularity--if you put the shift lever in 2 or 3, it will START in 2 or 3 and stay there; there is no way to start in 1 unless you have it in 1--not usually a circumstance that causes a problem, but something to be aware of.
It's a great truck and the TorqueShift works hard and long, just need to be aware how it shifts if you are using the options available.
Joe
By the way, went to AZ in Feb--all the way into 30+ headwinds, got an outstanding 6.0 mpg average for the westward trip. That was painful, and without the extra tank, would have been troublesome looking for fuel.