OK its like this. If you have a charging station you can charge from either side with the system OFF!!! In this state both sides of the system are the same pressure so it doesn't matter.
Actually in these type recharges the system is filled by driving liquid from a higher pressure vessel to one of lower pressure. This only works if you have a large reservoir of pressure not a single can. After charging in this method it would be dangerous to start the car immediately if one had charged through the low side; as there would be liquid refrigerant on the intake side of the compressor.
Most people (even with good charging stations) charge through the low side as it takes less time when there is a large pressure differential.
One thing you must remember is that the pressure of a container of refrigerant is only determined by the temperature not the quantity. ie: Five pounds of refrigerant in a thirty pound cylinder will have the same pressure as one pound in your inactive system. In order to get refrigerant into an inactive system you must fill from the bottom flowing liquid and measure the weight so you can stop way before the pressures equalize. If you were to fill from the top the pressures would equalize before a full charge occured.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician