Aluminium heads on an engine do not in anyway guarantee hardened exhaust valve seats sadly.....as many Rover V8 owners will be able to confirm sadly
That is not what the many Rover V8 owners I know say: you'll be suffering from knackered cams/followers/bores/ other low oil pressure related faults long before the valve seats fail. I read a letter from a manufacturer of valve seats some years ago, he reckoned that they had never(since the thirties!) sold a valve seat that wasn't suitable for unleaded fuel.
Pre 85 engines are well known for valve recession as they had a basic chilled Iron seat, this vastly increased if they operate on load for any period (so smaller cars not such an issue, Land Rovers etc, bigger problem).
There was always some confusion as Rover stated they were ok from the early seventies (they weren't but the recession was considered small.....), its when they finally opened the inlets up with EFi that they truly had to swap over to a proper hardened seat. Easy enough to track with the valve seat part numbers.