I think the cut down from the ribcage is sufficient, but it does have a downside. As time goes by, and especially if the weather is warm, the esophagus/tracheal area can really serve to spoil your meat quickly. The higher up you can sever both tubes, the better. It's easier to reach up there and sever them nice and high if the rib cage is opened up. A split rib cage also increases air circulation and cools the meat off faster. Some people I know don't cut all the way through the ribcage, but rather just go in (carefully) by the neck and make the cuts there. I don't think they do this on deer that they want head-mounts on, though.

Nate