I don't think there's a specific rule involved here.
It is perfectly fine to use up/down the street/road if the street is flat. Which of the two one would use is more debatable, and somewhat depends on each particular person's point of view.
For example, one way, as you've mentioned, is to chose by house numbering: the way in which the numbers are ascending can be considered "up the street", and the way in which they are descending can be considered "down the street".
Or maybe you can consider the direction in which the street is more... erm... fashionable to be "up", and the other one to go "down".
I would say that in general, if used unrelated to the elevation, these phrases can mean the same, and can be used depending on your own preferences. Also note that "down the street" will always be understood, which I believe is the reason for it's popularity comparing to "up the street".