Sorry, but you are wrong! A domestic socket only switches off the live circuit; if there is a wiring error somewhere it may be the neutral that is being switched off, leaving live and earth still connected to your PC. Play safe and unplug. You should bear in mind that the main thing is that you and the PC are at the same potential. If your PC is earthed, chances are that you are not, due to static from your clothes or carpet.
Simply touching the (unplugged) metal chassis is all that is required.