I'll bother "warming up" for precisely two reasons:
1.) it's seriously cold, as in single digits or below, which is pretty rare in Nashville, but it happens, or;
2.) my windshield is so frosted over that it takes a couple of minutes to get the heater going enough to start thawing it out.
Other than that, particularly since Nashville is usually a fairly mild clime, I'll usually just crank up and go.