At yesterday's Acoustical Kaleidoscope people reacted with surprise at the idea that they might play some of the most miraculous pages of piano writing by Beethoven without necessarily having to play the entire composition. It's funny how ingrained the notion is that a piece of music has a necessary beginning, when it is most obviously not a narrative. In fact, I believe many musical structures are flashbacks.
I let my young students start wherever they wish: find a spot you can play and work around it. Its amazing how that illuminates structure.