I, like many others rushed off and had hardened seats fitted when leaded petrol was no longer available. However, the ones that didn't ended up having no problems even after high mileages. Indeed, some are still riding around with 'soft' valve seats. My advise for what it is worth, would be to ride it and see if you are having to adjust the tappets more often than normal. If you have no excessive adjusting, harden the seats when the valve guides are shot. Additives are not my thing due to the fact they can bung up the carb.