The PC is still earthed regardless of the power switch, because the switch only changes the "live" wire
(Actually having said this, 2 pronged power plugs like http://www.leadsdirect.co.uk/images/...ng%20300px.gif
will NOT be earthed, only 3 prong such as http://www.leadsdirect.co.uk/images/...ng%20300px.gif
will be, and therefore 'safe')
It doesn't matter where in your house you plug it, they should all be properly earthed, in theory to the same place (some stake in the ground under your house probably).
TBH, I don't worry about it, I just make sure to not touch electrical bits without touching the case or PSU first.