Is what I'm doing OK?

[96-1500]

I do the same thing... When you think about it though I'm not so sure it actually hurts mpg's because the engine is wayyyyyyyy out of it's power band in overdrive. You'll notice that you actually back off the throttle when you shift down to hold the same speed because it's not lugging the engine anymore.

^ this. My K2500 is a heavy pig, and climbing steep hills at around 80 ish it likes to stay in overdrive but just barely...the engine is lugging and lugging between 15-1800 rpm and struggling to maintain speed. Shift it down to 3 and it's running over 2000 and much happier. I've never done any scientific testing but I would guess the mileage is better where it isn't working near as hard. Either way, better mileage or no, there is no question it's better for your truck. I run in 3 all the time and shift up to OD when I feel it to be necessary. I also shift down when slowing down to save my brakes.