I don't always start in the middle, but it seemed appropriate with this image.
It is the focal point of the whole picture with the area of greatest contrast and with the juiciest dark shadow. If I complete this section first I can make sure the other portions of the picture won't overshadow it.
To protect the ends I am not working on I have taped tracing paper to my drawing board. This means I can lean safely, without smudging the under-drawing.