Technically speaking, no one needs a teacher. But having an instructor or someone to guide you in your piano studies is definitely and exquisitely helpful. Having a piano teacher will widen your possibility in improving your skills, and fixing other things too. If you can't afford to pay a teacher, talk to a piano pedagogy at your local college. They can give you a few pointers and guide you in the right direction. Because, when learning piano, a teacher is there to help you embrace your skills and learning, but you are the one that chooses to practice and get better. A teacher can not force you to do anything. It's all about your decisions and the choices you make on how dedicated you are to piano.
So, I don't think you need one, but it would push your musical capabilities even further.
magic_sonata