LO? I build a lot of heads, porting, welding and re-shaping the ports, valve seats, guides, valve porting, springs etc. Most of the time the problem in racing scenarios is burned exhaust valves and the issues that come with that, worn guides, warped valve seats, low compression. Half the time you can correct some guide play by using new valves, the other half the damage is done so new guides get drove in, some people think it's no good, but I like using valve guide inserts, I've used them on a lot of engines and haven't had any issues with them. I like them because if you mess up the install on an insert, it's not a big deal, but with some heads if you mess up a guide install you're basically ruining the head. I always cringe when I have to drill out a guide, not to mention some guides are rather difficult to install to the proper install height. As far as valve seats, if they're beat up I'll drill them out and press in new seats and cut them, or I'll just re-cut the existing seat if it's still in good shape(ie no pitting, flaking, or noticeable deformation) and can get away with using an O.S. valve. Some people have the luxury of a CNC machine to do this, I'm a smaller shop so I cut all my valve angles by hand. I only send stuff out to a machine shop for decking or for drilling out valve seats, or for cylinder block work, but head work can almost entirely be done by hand.
Most of my builds are Hondas so all are OHC engines, Supertech makes very good guides, as does Ferrea, but for longevity nothing beats OEM valves springs and retainers for most applications, you only go titanium or uprated springs when you plan to increase the RPMs. Honda OEM valves are hard as diamonds, as long as the valve lash is kept within spec they'll easily last 100's of hours on track, as long as you don't do something stupid like a mechanical overrev.
You can always pull the exhaust manifold(s) to get a good look at the valves, as long as the leak down numbers are good I wouldn't worry.
The height of cultivation is really nothing special. It is merely simplicity; the ability to express the utmost with the minimum. Mr.Lee