It's worse for the engine to let it idle and warm up than to just start driving. Keep the revs under about 4k RPM until the engine hits its operating temp and you'll be fine. If you're driving it, it gets up to temperature quicker than if you just have it sit idling. Also just because your engine is warmer doesn't mean any other driveline components are up to temp.

The "old school" way of thinking is, unfortunately, horribly wrong for modern cars.