Relays and mains voltage (Europe 220V+US 120V)
Well, you don't have to blindly believe everything you are told on the Internet. Someone saying so can be wrong or just trolling.
Safest is to switch both live and neutral, because in some countries you can freely orient the mains plug in any orientation so you don't know which of the wires is live or neutral.
Another thing is if you plug the computer into old ungrounded plug, and only switch off the neutral wire by accident, you have a metal PC case connected to mains live via a capacitor, and that's dangerous.
Oh and old buildings do not necessarily have grounded outlets or RCDs so do not build anything that relies on the electrical system to be newly installed.