Yes, although I've never checked that out for myself, it makes sense. But just because the voltage is stable doesn't mean the engine is running at the optimum AFR for conditions or even the AFR that the ECM wants it at. In open loop the ECM doesn't know what the real AFR is. It's using it's other sensors and its maps to extrapolate a particular fuel flow rate. While this still may be pretty accurate, much better than a carb I'm sure, it's still not as accurate as direct feedback from the O2 sensor (Well, kind of direct. Scott explained it. You know what I mean, right?).