A default-converging asymmetry for a codebase: "Every time you touch an area of code, leave it a bit better than you found it."
The "leave it better than you found it" is in some ways a cheat because the definition of "better" is left undefined.
But as long as there's a consistent asymmetry in what people think 'better' means then there will be coherent movement in a good direction.
Even if people disagree on some aspects, there are others that everyone can agree on: tidy, robust, elegant, well-documented.
In many situations, people mostly agree on the general direction of better.
Rob Sand's slogan: "Not redder or bluer, but better and truer."