If I'm in the mood for it, not in a rush to go anywhere and if the system is not too big, but containing valuable planets and is not too large - then chances are higher for me to scan everything. If not...maybe the star, but that's it. If any gas-giants are in range - fine, these too. Earth-like and water-worlds go without saying.
For the current CG most of the time I didn't even look at the system map, I just "honked" and went on to the next system. As I've stayed less than 50ly from the bubble chances to get into an undiscovered system where pretty slim anyway.
However, in the past I stumbled over systems which I call to myself "Kilroy-was-here-systems". Usually these are not-too interesting systems (e.g. L- or M-star, icy bodies) without any gas giants/earth-likes/waterworlds. Usually it looks like this: star got tagged by Cmdr A, the first planet by explorer B, second planet by explorer C, etc.. Then I join the fun and tag the next planet, regardless what it is (ice, metallic, etc.).