Italic is like the Italic Rules of the internet: a font style so versatile, yet so frequently abused. When to use italic, and when to avoid it, is a font-asthetic question.
Use italic for emphasis, but not just for emphasis's sake. Use it for titles of books, movie names, and foreign words. But not for everything else. Don't be that guy who uses italics for every single thing.
Or, as Italic Guru would say, "Italics are like the cheese of font styles: a little goes a long way, or else you'll end up with a style disaster."
So, to sum it up, use italic with caution, lest you fall prey to the italic trap.