I can test this but I don't have my MPGuino assembled yet
I am running my block heater daily at this point on my VX. I always set the key to on and let the CEL and upshift light go out before cranking the motor over (this allows the fuel pump to prime and the car to start quickly). I have a yellowtop Optima deepcycle battery in my car (41Ahr) and can easily let the key sit in the "ON" position while I get my kids in the car and get them buckled and set to allow my Wideband to heat up.