There are 2 ways they may properly charge the system. Most likely they will simply weigh the charge in. That involves fully evacuating the system and then recharging it by the weight stamped on the side of the unit.
That procedure is not likely to be used unless the unit has been evacuated.
The other way involves charging by subcooling, which you already mentioned. If they try this in low ambient conditions, they will have to artificially raise the head pressure. Let the system heat up as it would in summer... They do this by covering the outdoor unit to restrict air flow. The house must also be above 70 degrees so that it gets a proper heat load when they switch on the air conditioning. Subcooling takes much longer and will be more expensive, but I personally prefer this method as it accounts for all of the system variables and ensures a proper charge.
Just heat up the house and put the unit in defrost which stops the fan eliminating the "need" to try to cover the outdoor coils. Defrost is the cooling cycle without the fan on the condenser running therefore raising the head pressure. It doesn't take long for that with no airflow across the coil.
If a system is overcharged, it will cause your electric usage to increase and your head pressures will be higher. The metering valve will prevent flood back of liquid to the compressor like you would have with a metered orifice system, but it would still not be ideal. If the system is undercharged, your TXV will not get enough liquid and the refrigerant will flash boil too soon in the evaporator. Your superheat will be too high and the compressor will run too hot, increasing wear and electric usage.
The tech will make the necessary adjustments to the refrigerant when he makes his checks.
The "exact" charge is really just somewhere in the middle where you have minimal head pressures and proper superheat. Your heat pump works in a variable environment so the exact charge needed for one day may be different than the charge needed for another day. Just as long as they don't have it drastically undercharged or overcharged, everything should be fine.
The unit has a chart or table to tell the tech what parameters for the conditions that day the unit should be.