Ethernet is electrically isolated at every port (unless you really go out of your way in to design it otherwise in an internal design). Your advice is still correct but it's more about lightning or something gone wrong, it's not an immediate risk.
I don't remember how it happened, but I have seen faulty Ethernet devices suffering a power surge that burned the switch port they were connected to, so be careful.
Inductance coils (usually inside the RJ45 jack itself) are what provides isolation. They are made from very thin wires so if something goes wrong this is the first and hopefully only thing that will break. If you apply enough voltage of course you can break things beyond this.
Most equipment connects the shield of all ethernet ports to chassis ground, so you're back to zero isolation if you use shielded cables.