I flew from an "information" ATS
From where I fly the AFIS want a call before you start taxiying, rather than before engine start. I was lead to believe that aircraft batteries were not that adept at supplying lots of electrical items and so you really don't want to switch the radio on until the alternator is running.
Certainly when I do my alpha check (pitot heater, landing light, stall warner etc) I don't leave these items on for very long as the engine is not running at this point.
Who knows about big 747's etc ?