I tought he meant case, from electricity in house, every component should be earthed that can potentially kill you.
Also bad wiring in house, corroded wires can lead to some instability to electric components.
Also that Ive seen more than enough , corroded wores are the most common problem in humidity.
Your pc is always grounded unless you isolate the contact from the plug, or plug a cable without grounding contact, which is sometimes helpfull in sound systems like rock concert, where if the electric amps are grounded can lead to humming or buzzing noise.
In short, in house if the electric worong is proper, and you use cable which was provided with your psu, there is nothing to worry about, PC is always grounded as the screws that...