What car? TR4? I looked at your profile and hope that my response is talking about the right car.
If we're talking TR4s, the heads do surprisingly well on unleaded gas. The metal Triumph used to cast them is more resistant to valve recession than some others.
Just last week, among other things, I was discussing unleaded conversion with Ken Gillanders, Triumph Frame and Engine. He's a master machinist and has been racing and building TR engines since these cars were new. In fact, I have to remember the next time I speak with him to ask if he still has his '55 TR2. At one time, it was believed to be the oldest TR still in the hands of the original purchaser. I trust Ken's opinion since it's based upon lots and lots of experience, his own and that of the folks at Triumphtune, Racestorations, Revington TR and other highly experienced TR tuners.
All in all, I wouldn't tear apart a TR4 engine just to make unleaded gas changes.
However, that said, I'll soon be doing a major rebuild on my '62 TR4's motor and will definitely put in exhaust valve seats. It just makes sense to spend a little extra to have the job out of the way and not have to pull the head again later to do it.
Possibly of more concern than the valve seats, you might want to consider replacing the valves themselves with improved, more unleaded compatible type. These come in both standard grade and more expensive Stellite faced (hardened). Also there are gas flowed and oversize varieties available. Increased hardening, gas flowed and oversize all add to the price, of course.
If we're talking about a TR4 #21,000 (approx) or earlier, the exhaust valves have triple springs and a 3/8" stem. There are easy conversions available for both, to 5/16" stem and double springs as used on later TR4/TR4A. The primary reason for doing this is a modest performance upgrade, but it's essentially "no cost" if valves and/or springs are being replaced anyway.
Due to the close positioning of the valves in the 4 cylinder head, it's not possible to run certain oversize valves with seat inserts, or it might only be possible to put exhaust inserts in.
Intake valves don't need inserts for unleaded purposes, but might for repair or excessive wear. So, in part, the condition of the head might dictate what changes can and cannot be made.
I'd definitely suggest using the bronze valve guides in the 4 cylinder head. These provide better self lubricating properties that's necessary without the lubricating properties of leaded fuel.
If yours is an early 4 cylinder head, CT21000 approx or earlier, it uses a .55 outside diameter exhaust guide and a .50 intake guide. As already mentioned, it's easy to convert to a 5/16" stem on all valves, with the correct exhaust guides. Or, if you wish you can retain the 3/8" exhaust valve stem.
Perhaps you've already considered these, but here are a few more things to think about regarding the rest of the valve train:
Are you doing any work elsewhere? Cam, followers, pushrods, rockers, springs?
Briefly, cams can be hardened to help reduce wear, especially during break in. Properly lubricated and broken in unhardened cams seem to last pretty well, though. If using harder cam followers, a hardened cam becomes more critical. Ken now offers GT40 followers with special shims that seem to solve many problems. In the past, various hard metal followers were tried and largely just accelerated cam lobe wear.
For my car, I've got a set of standard size, phosphated followers that will be used with a freshly re-ground and hardened, slightly-more-aggressive-than-stock cam shaft.
Also consider what pushrods you'll use. Tubular are available, are stronger than stock, plus are lighter and less prone to flexing. They can also be more easily shortened to the exact requirements of an engine that's had the head skimmed a bit to increase compression (my car's head has been trimmed .125"). Tubular pushrods are highly recommended!
The rockers themselves should be refaced. That's precision machining to put just the right shape on the end that wipes the valve stem end, to keep wear and sideways pressure to a minimum. (I plan to use roller rockers for this reason, that's another alternative.)
Check for any play in the rocker shaft and/or rocker bushings that ride on it. Replace any parts with wear, or abnormal movement can make short work of new valve guides.
I understand there are some pretty poor replacement valve springs out there right now, for 4 cylinder TRs. Apparently they are pretty "soft" and mismatched, even brand new from the box. Watch out and have any you buy tested.
Many 4 cylinder TR racers question the need for heavy duty valve springs. There seems to be little or no problem with the stock rated springs, even at high rpm. Heavier springs are used in some modified engines to prevent "valve bounce". But, that doesn't seem to be a problem with TR motors. Heavier springs seem to just increase wear on cam lobes.
It is also important to watch out for valve spring bind. That's especially important with increased lift cams and/or higher ratio rockers. Stock ratio is something like 1.45:1, although 1.5:1 is often considered stock replacement. Some are as much as 1.6:1 and might lead to problems. Generally consensus seems to be to stick close to stock with 1.5:1 and have any increase ground into the cam. Still, there is apparently only a few mm of free space in stock type springs, so they should be checked against any increased valve movement.
Hope this helps!
Alan