My factory dual fuel, and the aftermarket dual fuel on my fairmont, both automatically start on petrol regardless of what fuel is selected. If its on gas, it switches to gas automatically as soon as it fires up.
If there is no petrol in the tank, the car will start on gas, it just takes longer. Doesn't do the fuel pump any good though, because it continues to run, dry, so you should avoid ever running low on petrol if possible.
Don't know about shutting it down on petrol though, never heard of that and cant see how the car could do it automatically either, it cant predict when you are going to turn the key off ..
If you are supposed to do it, I never have, and I've done maybe half a million k's on gas in several cars with no issues.
With my current VS Berlina with factory dual fuel, I have to come to a complete stop to change fuels ( but I can leave the motor running, just keep the throttle up a bit while I hit the switch ).
In my EL Fairmont with aftermarket gas, you can do it anytime. I prefer that... if the car gives a cough running out of fuel just hit the button and switch fuels, no big deal. The factory setup in the Berlina means I have to actually stop first.
Gas either works or it doesn't too, so there is no warning, it just dies. With the aftermarket gas if you can hit the switch within a couple seconds the car will keep going, but with the factory version that's not possible. I would have to coast to a stop, then switch fuels and start it up again.
The plus side of factory gas though is that I only have one fuel gauge, which displays whichever fuel I am using at the time, and the trip computer still works accurately regardless of which fuel I am on.
Using a tank of petrol every few tanks? My Fairmont has 500k on the clock, 400k of that with dual fuel, and its never had petrol added unless it was real low, either from use starting or having used it as emergency fuel. I doubt I would use a tank of petrol more than once every 6 months. The old 4L is still running well. No, it wasn't a taxi either, all the k's are mine
When I do use petrol though, it works just fine. Never had a problem.