Rule 1: Italics are for when you're feeling fancy, or when you want to convey prophets of doom, or when you're trying to make your cat's name sound cool. Read more about the history of italics.
ประกRule 2: Use them for emphasis, but only if you're a typographic hero. Otherwise, you're just shouting in the digital void.
Rule 3: Never use italics for idiots or those who can't even be bothered. That's just a waste of everyone's time.
Rule 4: Learn the rules of engagement for italics, or you'll end up like these people.
Rule 5: Sabotage the rules with your own italic style, and watch the world burn (in a good way, hopefully).