I use it on hills or if I'm towing something.
It won't hurt the engine/transmission to use the tow haul(T/H) mode all of the time. Basically it keeps the trans from shifting into overdrive. Overdrive provides better gas mileage on mostly level roads.
If you always use it you may use more fuel. Try a couple of test runs with it engaged and calculate your mileage and then try the same route and only use the T/H mode for up or down hills, compare your mileage.