First of all, how can you be so sure of that, Nikotine? Have you ever tried to write a song?
Yes I have. In fact, I am doing just that right this moment!

Second, I don't recall ever hearing Miley use auto-tune yet, but even if she did, no talented singer uses auto-tune all the time. They might use it to add some sort of audiovisual special effects, but that's about it. I'm sure you can tell when a singer is using auto-tune and when they're just using their regular voice, because when one is using auto-tune, you can hear some sort of special effect in their voice.
The reason I say this is because I once watched a youtube clip of her singing party in the USA live and it was so bad I laughed the entire way through. It literally sounded like screeching.

However I admit that while auto-tune pretty much seals your fate as a crappy singer, it's totally okay for a composer to use. If I were to ever write a song that involved my own voice I would have no choice but to use auto-tune as I can't sing at all. So if she really writes her own songs and if she has written songs with an actual chord progression I can't knock her for using auto-tune as a composer. It's so far away from my style of music, though, that I would rather not subject myself to the rest of her oeuvre just to see if she does, indeed, use chords or not.