Let me mention that, in addition to those unfortunate enough to experience actual failure, seizure, excess wear, etc. in valve gear, there are also less-than-perfect installations that may show no immediate signs of error but in which minor tweaks will provide more power, more stable high RPM, etc.
Whenever you have it apart, take a moment to analyze anything that "doesn't look right" (scuff marks, dissimilar wear patterns on stem tips, burned/blue pushrod balls and cups, fasteners that continue to loosen, valve adjustment that changes over time, etc.). Any anomaly is cause for inspection and discussion.
These archives contain a huge volume of experience, real world data, history of experiments, etc. well worth your time to research.