I have just bought a car with an accessories socket (White Cap), and something in the back of my mind tells me that I need the engine running before I connect the plug. Is this true, an old wives tale, or am I just getting too old ?
Thanks in advance for any answers.
Depends, some cars have them live with the ignition and some have them live 24/7.
On my car moving the fuse in the slot changes from one to the other (Mondeo). In the upper position its live with the ignition and move it down and its live all the time.
Its an old wives tale, it really does not matter. Having said that, it is probably better to connect without the engine running as most of the live pins are (or should be if correctly wired) powered up only when the engine runs.