I only replace things if they are worn beyond tolerance, leaking (seals/gaskets,) or if they are one time use items (TTY bolts, etc.) Anything beyond that is unnecessary.

I'm with you on that since I too am an engineer but the wallet has a say too. For example, valves never need replacing, unless you kiss the pistons w/ them, rockers the same, unless the bearing goes (which isn't often). Though yes, I'd replace them too if better parts are available. The problem is for the valvetrain, there isn't anything that's really better (adjustable cam gears on the Mod engines are a horrible design) or it costs an arm and a leg (Jesel lifters). This excludes the cam though.