I take it back - it's not "Never". It's "user discretion advised".
If you're doing just a shoulder+head portrait then yes, stay in the same plane as the face. If you're including more torso then you need to lower the camera, but ask the subject to lower their chin just a bit.
Again, this is not a black and white situation. It's a rule of thumb. For a person with a longer nose you might want to shoot from a lowish point - to compensate and "shorten" it.
As with everything else in photography and in life - you need to be aware of it, and experiment. Not do what others do.
Eventually you'll figure out what works.