I checked the POH for a 1967 Cessna F172H and it says “Just before connecting an external power source (generator type or battery cart) the master switch should be turned “ON””
It also goes on to say that the external power cannot be used to power the onboard electronics only the aircraft electrics. The bus bar is split and the electronics bar is isolated during external power provision to protect any semiconductors in the electronics from damage by transients in the external power source.