Where I used to work, we normally left our portable oxygen bottles in the following condition: valve off, regulator attached, flow set to zero. The only time we would turn the oxygen bottle valve on would be when we were actually using it or when we were checking the pressure in the bottle. Since we always had our portable oxygen bottles with us, if we needed to use the oxygen bottle, we simply had to turn on the valve and set the flow to the rate desired. This prevented leakage of oxygen from the bottle and thus prevented us from having an empty oxygen bottle.
The main oxygen bottle on the truck was normally turned on and checked, flow rates of the regulator set to zero, and left in that state. At the end of the shift, we would turn the main valve off and bleed the line out. We would also do the same with our portable oxygen bottles, valve off, regulator bled out. That was to keep high-pressure gas from staying in the system and giving us a false reading.
If we forgot to turn the main valve off and bleed out vehicle's system, we might only get into trouble if the main O2 tanks ran dry over night. Maybe. If it became a habit.