I used the full 15, and I spread them pretty evenly between employment, community service (clinical and non-clinical), teaching, awards, research (pubs, presentations), and others (I enjoy running, sailing, rock climbing).
I reasoned that since the ADCOMs can probably only give a score (say 1-5) in any of these categories. I felt that I would enhance my chances if I recieved some score in all of the categories, instead of focusing on one category, get a high score, but zeroes everywhere else.
This has another advantage - you look more balanced, more experienced, and you're more likely to make a connection with one of your interviewers (which they'll use to learn more about you).