I am installing a new PSU in a computer, the PSU - Tagan - has a grounding wire, which the manual indicates should be attached to the motherboard via one of the screws that keep it in place. Have never come across this before, is it necessary/desirable/a good idea?
It is not neccessary or recommended to fit a seperate earth connection if it was was not installed by the manufacurer. Earthing a system not designed to be at earth potential could be detrimental to your system.
A power supply with a spare earth is designed for a system that specifically requires it.
Any changes such as this should be referred to the computer manufacturer.
"Earthing a system not designed to be at earth potential could be detrimental to your system."
Whilst that is true - in a computer system the earth plane on the motherboard and the case, and the PSU are all supposed to be connectd together and be at ground, and earthed via the mains plug.
"Ground rule for most electronic (and electrical) equipment Earth at one point only"
Motherboards are expected to be earthed at multiple points. Each mounting point is connected through a metal stand-off to the case, the plug from the PSU to the motherboard also contains several earth wires, and there are earth links throughout the case, the plug-in boards, the drives etc. Multiple earth points are common and expected in very high frequency systems, in order to minimise (electrical) noise.