The sterotypical rule with overdone songs is that you have to sing it at least as good as the orginal or better. If you can not sing defying gravity better than Idina, stephanie, shoshana, or Ana , then don't bother. Same goes for Millie if you and everybody else knows you're a better singer than sutton than maybe you could pull it off.
Usually even if you are better, the director might not see past the song to actually realize that you are more talented that Sutton or Idina. So ususally, that's not going to happen.
plus, you want to stand out of the audition crowd. So why sing something really diffrent.
"You gotta be original, because if you're like someone else, what do they need you for?" -Bernadette Peters
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