There is some confusion about when hard seats are needed.
If an engine is run at full throttle for extended periods, as in pulling a heavy load over a pass, or a tractor pulling a plow for extended periods, the exhaust valves will become red hot. At this point when the valve closes it welds it's self to the valve seat. Then the next time the valve opens it breaks the weld and a little bit of metal is removed from the seat. Over time this causes the valve and seat to become recessed in the head. On a mechanical lifter engine this causes the rocker arm to valve clearance to go away so the valve can not close completely and eventually the valve will burn. On a hydraulic lifter engine the lifter will compensate for this wear for a while but over time the same thing will happen.
Another time a problem can arise is when an engine is timed too slow (retarded). In this case the air fuel mixture does not have enough time to burn in the power stroke so it is still burning as it is going out past the exhaust valves on the exhaust stroke. This will also super heat them and cause welding.
The lead additive in the past acted as a barrier between the valve and the seat preventing this welding action from taking place.
Valves do not like to weld to hard seats, being a dissimilar metal, so that is why they are sometimes used.
One other option that I use just for cheap insurance during a rebuild is to use stainless steel exhaust valves. Stainless steel being a dissimilar metal does not like to weld to cast iron either.
Chevrolet six cylinders from 1950 on use the same exhaust valve as the small block Chevrolet so stainless valves are cheap and easy to find. In most other cases stainless valves are available from places like Egge Machine.
There is one occasion where hard seats are needed and that is when the seats are eroded away from this welding action or from repeated grinding. This will leave the valves recessed in the head causing restricted flow in or out of the engine. This will effect engine performance.
I do not like using hard seats unless absolutely necessary as I have seen them come loose and damage an engine and they are fairly expensive to have installed.
The bottom line is if the engine is timed correctly and running fine and the engine is not loaded heavy nothing special needs to be done.