Nodrawing faces that are indeed drawn by the engine (but not visible because there will always be obstacles in the way) will improve performance, yes. But, this is a fairly rare situation : it occurrs with world-sealing brushes behind displacements or very large props (such as the 2fort rock wall). Usually, the entire brush will be textured with nodraw anyways (not just specific faces).
Nodrawing faces that are not drawn by the engine, typically the back sides of details that are in "eye candy" areas (not accessible in normal game play), will also improve performance, but as Tony says, by a very small margin. To put things simply: you should do it, but it doesn't really matter.