One general piece of advice I've heard is that ski length should vary with your height, but ski stiffness should vary with your weight. If you weigh more, you can more easily bend a stiff ski at lower speeds, and you may overpower a soft ski on hard snow.
Of course, it's not always that simple, because stiffness also tends to vary with the length of the ski, and there's no simple uniform measure for 'stiffness'. (A lot depends on whether the ski stiffens gradually or suddenly as it bends, and the balance between longitudinal and torsional stiffness is also critical to the 'feel'.) You can also get into distinctions between the 'length' of a ski (tip to tail) and the actual running length of the ski's edge. If two skis are the same 'length', the one with the shorter radius/skinnier waist actually has longer edges. Rockered designs also effectively ski 'shorter', since more of the forebody of the ski is off the snow at the start of a turn.
IMO the advice of not going much beyond head height makes sense, at least for frontside-oriented skis. Because of the physics of applying leverage to the skis, it can be quite difficult to pressure the front of a ski that is significantly longer than you are tall. But some of it depends on biomechanics -- things like how long your legs are relative to your height, for instance.
FWIW, I'm about 6'6" (78" ~= 198cm) and I don't have a problem on 170-175cm carvers -- if they're stout! I actually went with a 175cm Fischer Progressor 9+ rather than a 180 because I felt it was plenty stable at 175 and I wanted better maneuverability in the bumps.
Ski what you like. If you like the feel of a 168cm ski rather than 160 or 163, go with that.