hehe, I'm working on the Toccata from Partita No. 6, and this is what my teacher told me: the trills begin on the dissonance (like, look at the chord in the other hand) -so in my case that would be starting on the written note, then going up... but then we listened to Glenn Gould's recording and he started a note higher than the written note, on the , uh, not-dissonance? hehe, so there you are, my teacher's like, if he can get away with it, you can too, but I suppose I think the first one makes sense...