Depends on how you want to drive it, and what fuel or additives you run through the engine. Unleaded fuel only cause issues on the exhaust side, no need to install seats on the intake side. What happens is that the valves and seats on the exhaust run hot enough that the valve will pull microscopic bits of metal off the the seat, and eventually you will get what is called valve reversion. The seat sinks in the head. If you run race fuel it has lead in it and fuel additives that have lead in them lubricates the valve face so it will not pull out metal from the seats. also exhaust temps have a big part to play in how much reversion or sinking of the seats, and how fast you will see it. Higher performance and lean fuel ratios advanced timing all will raise exhaust temps. I tell people if you have a show car and drive a couple thousand miles a year, just dump some additive in every once and a while. If you have a race car and run race fuel, don't worry about it. If you want to take your car on the hot rod power tour, then get some hard seats installed. All stock iron heads after about 1974 have flame hardened seats which is supposed to fix the reversion issue, it does NOT, I have torn apart many newer engines with flame hardened seats and many high mileage engines have seat reversion. Hard nickel chrome seats like are used in aluminum heads and retrofitting older cast iron heads solves this issue. One thing is that in a performance vehicle where you want all the flow you can get, sometimes the shape of the hardened seat may cause some issues in the port and reduce flow, so you may loose some performance. I say this because many machine shops will simply cut the old seat and press in the ring and leave a pretty good ridge in the port. A little die grinder work will help, but does not always fix the issue as it really changes the shape of the short turn radius.
