Regardless of the size of the map, I will not add extra cities just to fill up space. There is a diminishing return on investment for extra cities in BNW, especially for science victories.
For example, I did a duel with a friend to see who could build the "better Poland." I ended up making 12 cities, but for the game settings and conditions, I think 8 cities would have functioned better. By the time the game ended, I think the tech/policy/gold and opportunity cost (when you build a settler you don't grow and you are not building something else) for the last cities didn't give me enough reward for my costs. If the game would have lasted another 50 turns then I think for sure the cities would have contributed.
Unless your goal is to play a game to maximize points. Then I would say you are in business!