that makes me cray..okay crazier.
If you don't know when to use it then don't. If you have to ask...don't use it. Only very good designers can play with that tool outside the rules and make it work.
Don't look at me, I am not one of them. I stay away from the justified text button :D
When you use it, where you should not, your layout looks amateur - nothing will show off a newbie designer like poorly layed out copy. (And using Microsoft Word Art...or an overused font...or a...ah....I better stop now.)
I am currently reading a book that I like very much...but the layout (which uses justified type) is distracting.
Incorrectly used justified type is a rookie mistake....don't make it.