I find this question slightly misleading and overgeneralizing. I have to assume, given the wording and the context of this website, that "Music Theory" refers to Western Tonal Music. This is a major overgeneralization, since Western Tonal Music is such a minuscule fraction of all the music ever created on Earth. I almost ALWAYS consider musical theories when I compose music, and simultaneously, I ALWAYS use my inner ear, the musical ear. But only every now and then is the theory I try to apply Western and/or Tonal, so I responded moderately to this post.