It's a valid (and important) question. If charging by refrigerant weight, you may not even need to have the vehicle running (depending upon temperatures). If charging by system performance, both halves of the system should be operating. Been there - I have two Chevy vans with dual air, and one has lost belly seals on the compressor once already.
Hopefully the proper volume of oil has already been added to the system as well as the compressor - That question has not come up yet.