I do it religiously. Half of my 20 mile commute is 45 mph on open road, and cruising steady state at 45mph with OD off definitely improved fuel mileage. Stop and go in town doesn't really make a difference.
I end up using it like a shifter button. When I hit one long downhill I turn OD back on and basically let it coast, then when I go back up the other side turn it off again so it's in lockup climbing the hill.
Also fun when you get on the interstate, let that beast wind up to hit 65, then turn OD back on.
