the players make the map, and if they are making an imbalanced map, that is their fault (except for the person who is choosing his starting location first, in which case he is playing strategically)
Each player should be placing map pieces in such a way as to make sure that the map benefits himself the most. This might mean making all the starting locations as equal as possible.
I think that when you first start, you dont know what is a "balanced" map. But with experience, this will become apparent. While city hexes seem nice, they also have **** resources, so they are not by definition "better" than a non city hex. Lots of cities wont help you build your strong hex units, or feed your armies.