There was a thread somewhere about 'what would you say to the Buddha if you met him on the street' -- getting clarification on this issue is near the top of my list.
I've never tried explicitly slowing my breath because my "shifting the mind into neutral" always slowed and lengthened my breathing naturally. I've also found it feels quite natural for me to define the phrase "whole (breath) body" as the sensations throughout the area affected by the breathing -- i.e., the entire nose, throat, brachial passages, etc. For me, the initial concentration on the tip of the nostrils, etc, just flows naturally into a larger awareness of all the body parts involved, including the indirect motion through the shoulders, arms, etc.
With regards to literal whole-body awareness, I find it a fascinating object of observation. I used to trigger it while driving by focusing piece-by-piece on the contact of my clothing. I'd do a whole-body scan section by section deliberately sensing where my clothes make contact. By the time I had cycled through all the touch-points, it was usually pretty easy to morph into the whole-body awareness. These days, I find it easiest with my walking meditation.