Dumb question...is it better to drive with O/D always on?
Most of the time, yes. You shouldn't drive above 60mph for long with it off, just for a short burst, like getting on the hwy or the problem in this thread.
I would turn it off when I was driving under 60 and when I need it to downshift for a little extra power. This is not saying much I got rid of my auto for a 5 speed.
Keep in mind when you turn your car on your OD is on by defalt even if you turned it off when you shut it down. So there is no way you could forget and leave it off.