I've never used the O/D button unless the grade is really steep going up. But the PCM will have already downshifted for me. If I'm going downhill, I might punch the O/D button to get the rpm's up to get more stopping force from my exhaust brake.
I think the most important issue to be brought up here is tranny temps. The factory tranny coolers were lacking for the 7.3. The first ones were just plain too small and did not have a liquid cooler inside the radiator. Ford changed that I think sometime in the 2000 model year and came out with a service bulletin to change the radiator for earlier trucks.
Many of us with 7.3's made our own improvements in the tranny cooler area. One of the first improvements was to remove the factory tranny cooler and replace it with the one used in the V-8/V-10 gas engines. It was about as close to a bolt in job as one could get. The routing for the tranny cooler lines had to be tweeked some to clear the added cooler height. It was around 1/3 bigger.
When the 6.0's started coming out, the tranny cooler was yet even larger by a group. It too was boltable right in, but because it was taller, the cooler lines had to be modified.
Since I had already installed the cooler used in a gas engine, I added an auxillary cooler that was about 14" square. I think about 190* is about the hottest I've ever seen my tranny temp get on a very warm day.
The general rule IIRC, is for every 10* you can reduce the tranny fluid temp, you double the life expectancy of your tranny.