Sometimes the valve stem settles to the bottom and sometimes not. Not only does the material removed from the rim at least partially offset the additional weight of the valve stem, but tires and even rims can be unintentionally weighted unevenly. I've had a wheel/tire (with no reflector or computer magnet) that, if positioned so the valve stem is at the bottom, will actually slowly rotate so that the valve stem ends up in the 2:00 or 3:00 position. I don't know if the wheel is unevenly weighted or the tire, but there's weight somewhere that will eventually settle to the bottom.

I know this is only partially related to the original topic, but I would say that you can tell if your hubs are in good shape when a spinning wheel settles to about the same position each time it's spun.