If you are intending to open the case of the computer then it is always preferable to switch off the system at the wall socket and remove the plug.
Switching off the switch on the power supply unit will still allow mains voltage into the case, where it connects to the switch itself

Note
When the system is connected to the mains supply by a three pin "earthed" supply, leaving the wall socket connected but with the supply off will provide some protection against "static" damage to internal components when working inside the case. In these cases the power supply should be "locked Off" where possible