If I know it's not going to dip below 60*, I'm okay with not taking under insulation (though my current set-up has me carrying the things necessary to make under insulation if needed; my poncho doubles as the outside of a Garlington taco). Buuut...I'm a nearly-inhumanly warm sleeper (I once took a space blanket and my poncho as under insulation down to 43* F without freezing to death).

Most folks find that they get cold without bottom insulation somewhere between 70* and 75* F. YMMV, though.