Reving it up a few times is the worst thing you can do, far better to let it sit at idle first, need to give the engine time to get the oil up to the top of the engine before reving it.
As far as why the change from the old prinicple of always let it warm first, a lot has to do with improvements in oil, it doesn't take as much now to get it flowing, due to technological improvements, as it did when that old axiom originated