Originally Posted by SylenThunder
The reason we say not to do it with the car running is because an unwanted surge of power when you flip that switch could destroy your ECM and render your car useless.
I see this as 100% conjecture. Who is "we" and is there any actual empirical evidence or design knowledge to suggest this is the case? For instance, turning on a lamp when the wall switch is on is no different than turning the lamp on before the wall switch.
Better safe than sorry, sure. But it's conjecture unless "we" knows something more.