Power Off, Safety Question

For many years when changing cables/ICs between components
I have followed manufacturer manual advice and turned main
power switch on units off. No mention was made about also unpluging units so I just turn them off, but left them plugged into power source. Is there any reason why units
should be turned off "and" unplugged or is this overkill
and unnecessary?

A related question when changing cables between two components does entire system have to be turned off or just
the two components getting the cable change?

I confess little electrical design knowledge but would appreciate members thoughts on this.
I'd say that just turning them off would be fine. You are disconnecting the power either way. If you were to open up the chasis for any reason i'd definitely unplug them as you'll still have juice right up to the power supply. When you are switching interconnects I'd recommend shutting off all of your equipment, not just the specific components. All it takes is one miscue and you could have a huge amp and speaker damaging hum. Take the extra few seconds and turn everything off to protect your equipment.
I agree with Grumpybb and would add that when you power down you should turn off your pwr amp off first and then everything else. If your pre has a mute switch use it as well. Reverse everything when you pwr up again.Good listening!