Hi Drew
When you select an expansion valve you select it by Capacity (Kw), Evap Temperature, Condensing Temperature, Sub cooling (K) and refrigerant type.
So you then select the valve that suits the above design criteria. Almost in all cases the expansion valves will be either just the right capacity or slightly oversized. To select a valve that is to small would cause poor duties low suction conditions and poor evaporator control.
So the expansion valve should always be able to overfill the evaporator which is where 'Hunting' comes in. If the valves superheat setting or the installation isn't as it should be for what ever reason the valve can over inject then the valve closes due to low superheat , the valve then stays closed until the liquid has started to evaporate and become superheated for the valve to start to open again hence 'Hunting' occurs. So every expansion valve even though it comes 'Factory Set' must be adjusted to suit the plant its fitted to, which is why they have a superheat adjustment to increase or decrease the orifice spring tension. As you say approx. 6 Deg K superheat is a good general rule.
If the valve is undersize it can never hunt as it can never overfeed the evaporator to cause liquid to exit into the suction line.
The problem with Thermostatic valves is that they have no brain and they can be opened and closed by the change in superheat detected by the bulbs temperature/pressure, as design, but they can also be opened and closed by compressors loading and unloading, as the suction pressure changes with the compressor load patterns this can cause the TEV to over inject as the pressure in the equalisation line is a closing force for the TEV, if that pressure suddenly falls as the compressor loads up, it forces the TEV to go open , it will eventually recover its superheat control but this can cause liquid slugs etc.
TEVs are the most misunderstood control in a fridge plant yet they are one of the simplest. Kr Glenn