A simple rule to null, texture every face (with null) a player never could see ingame.
Although it CAN cause problems, under certain circumstances!
While developing my canyon_redux map, I started having bad lagging problems which I finally traced back to a particular set of NULL surfaces. They couldn't be seen by the player, but they seemed to allow the engine to see out through the void and back into another section of the level. Whenever you were near that other section (which was, itself, rather complex) the game would lag horribly. I went through, grouping and turning off all sorts of details to no avail. Finally, I set those NULL textures back to standard wall textures, turned everything else back on, and it worked fine...
YMMV!