Nope, not having that. I do everything by ear (but I usually know how to play a song after hearing it by practising it on the piano for 10 minutes or so). That said, I can't really plan on how the mix is going to be before I start playing. I sometimes have a very good idea what feeling I want to convey, what vibe and emotion it needs to have, but other than that I just sit down and begin to play.
After I have the "skeleton" of the mix in piano form, I start adding extra instrument, redo sections, tweak stuff, etc