My salesguy said he was told to hold the Start button down fully, rather than just punch it...it's supposed to make sure the electronics fully load. But he's a salesguy, probably trying to head off a complaint about the system not always loading properly (mostly XM fail-to-load issues).
That's for sure. I've hit the button to shut off the car and come back to it hours later with the engine running. You really need to press it fully and then watch the tach drop to zero because it idles so quietly