I don't turn the OD on until I reach 55mph on the highway. My Trac shifts into OD at 45mph (which I hit in the city all the time) when the OD is left on. I don't like driving below 2000 RPM if I can help it - sounds like the engine is "under load" at that RPM.
Just a matter of personal taste, really. I'm sure there's no harm in leaving the OD on all the time - except when towing.