I am with Matt. Supposing you really want to use 1/40 @ f/5,6, you consider it 1/3 EV closer than 1/30 @ f/5,6, e.g. you just turn the aperture ring "a little bit" past the 5,6 value. With most lenses you are not bound to the "click positions" of the aperture ring. The aperture ring moves continuously, and the clicks are only there for convenience: in the old times, when shooting manual and having no indication in the viewfinder, you could modify exposure by just knowing how your camera is set and "counting" the clicks, without moving the eyes from the viewfinder. For street shooting, having click positions means you can modify speed and aperture without looking at your camera, while keeping it in your hands, before bringing it to your eyes.
Older lenses do not have clicks and diaphragm moves continuosly. Still older cameras don't have clicks even on the shutter selector. While I would advice against using "half positions" for shutter speeds, I think that using half positions for apertures is safe with MOST lenses.
Fabrizio