Actually, it seems like it works the other way around -- the more advanced you are, the more important it is to have a good teacher. You can possibly teach yourself elementary technique, but if you really want to sound good, a teacher can make all the difference.
That's why you so often hear about famous pianists, that they studied with a student of a student of a student of Liszt, or whatever. Even Murray Perahia, who apparently didn't take lessons till he was in college, studied with Mieczysław Horszowski and Rudolf Serkin eventually.
In your case, if you're already working on Chopin etudes and Liszt, now's the time to find somebody really good.