Do you mean that you exclude walnut and other species that don't belong in compost? That strikes me as a good idea, but I don't know how much it matters, given the volumes of waste we're talking about. A few gallons of walnut shavings probably wouldn't make a difference when mixed with a whole city's grass clippings. Of course, chemically treated wood would be a different story. I don't use much walnut, so it's a pretty academic problem for me.