If its a tube and fin radiator I would put a pressurized cap on it. If its a soldered honeycomb, then no. Back in the day ppl would check the coolant levels more frequently, so topping up was a common thing. Now a days most ppl don't know where the radiator cap is, or worse, add coolant to the system through it.
You can lose coolant just by steam loss. Unless you are in the habit of a thorough 'pre flight' you may learn you are to low on coolant far from a source of water.
Putting a pressurized cap seals the system and keep the water from steaming out yet allows it to vent if needed.
I say add a 4lb. cap. You don't NEED to, but the benefits out weight the risks.
