Use it sparingly, just a touch or you'll get that underwater sound like on the old 60s song "Moles". It's very tempting to overdo chorus on vocals because there seems to be a fine line between when you can hear the effect and when you can't. But you should apply it until you can hear it almost ridiculously, then start backing off until you can barely hear it. Then back it off a touch from there. It may seem like nothing is happening, but depending on your mix, it'll be having it's effect. I've learned my lessons from past chorus overkill and unless you specifically want an over the top effect, it sounds awful and you'll probably regret it once the initial novelty has worn off.
As an aside, I find that delay does a better job of thickening vocals. In any case, you'll need to experiment with the three settings to see how each affects the sound and mess about until you find what you like.