As you're playing games or working, the only time you ever turn your head is when something external distracts you, and you can resolve it in a split second. If you're using VR and something external distracts you, you have to abandon whatever your doing and dismount the device.
i don't think physical movement will be a requirement of vr as a concept. you can sit and eat cheetos all day long if you want.
and in the future, i think we'll have instant-on instant-off see through LEDs so you can hit a button on the side and see through the headset into reality.
Unless you wear glasses.
As to the original point, people adjust their head position all the time without even thinking about it. It's an unnatural and conditioned response to hear a noise on your right rear and move a joystick or mouse to look for the source of the noise.