IF the texture is made with overlaying it over an AO bake, how does that impede adding AO another time? Surely you can still see the AO below the grime on the texture.
If I understand your question correctly, the overlaid grime/AO layer doesn't prevent you from adding another AO layer if you wanted. Personally I don't usually use a regular AO layer because it's purpose is to add contact shadow. However being a realistic GI engine, Cycles does contact shadows automatically, thus nullifying the need for a regular AO layer in the shader. It's amazing for dirt/dust/grime though.
