What a fantastic thread! I forgot how fastidious owners can become - my bikes are always second hand so I'm pretty sure, knowing how life deals me cards, that some previous owner had enjoyed banging the engine on its rev limiter every time he saw a bloke he fancied & hoped to impress so my leaving the valves for another few years won't upset things... being serious for a moment checking the valves' clearances on these bikes are a tad more time consuming than checking your pubes for lice, how many folk actually see the clearances changing over time? Or do non of us ride enough to actually cause any change? I'm of the mind that the valve train will settle after the first say 10000 miles then remain in a steady state unless the engine gives cause for wear (running hot, sustained high revs etc.). On the helmet thing I would, on this topic, change sides and err on the side of caution... every thing complex on this dear planet tends to degrade to a simpler form over time, be it life itself, a radioactive nugget or even, my dear bikers, a helmet. The integrity and performance of your helmet depends very much on the properties of each of its systems - from the laquer that protects the integrity of the shell, to the padding that snugs your helmet to your head. Over time degradation of each layer will contribute to reduced performance should your lid be called in to do its job - some parts such as the lining will degrade much quicker through use whilst other parts like the shell will change its physical properties through attack by UV light, heat cycles etc even if not being used. By all means wear a favourite lid whose lining is so comfy (because it has been squashed in to the shape of your noggin over the years) but don't expect it to grip you like it did when new or offer the same force retarding ability when it is half as thick as it should be. The shell, depending on the material used, can become fragile like an egg shell over time, it's supposed to prevent puncture and spreads impact force over a larger area, attacked by sun light and chemicals it comes in to contact with, or by doing its job and banging in to things like your hall table or your CX500 exhaust pipe - and the thick absorbent material that forms the bulk between the skin and the padding is designed to absorb energy by deforming (a bit like a crash zone in a car), a lid can look perfect but underneath the shell will be areas of compression of the absorbing zone where the lid has met an object (eg the ground) with sufficient force (they are supposed to deform if they meet a force greater than the equivalent of dropping the helmet from about 1 metre) - clearly once compressed these areas don't rspring back to their original shape and can't really compress much further. Every time you accidentally drop your lid you may be creating a patch of helmet that will offer little protection should it be the point of impact in a spill. Speaking of someone who suffered a massive head injury back in the day, probably saved from death by a newish Nolan N42... please dear bikers replace your lid periodically (the industry and our glorious government recommend every 5 years) or should you ever have even a minor spill - you can't know for sure you didn't bang it as you slid over the run off zone and if you did the helmet may show no evidence yet underneath the shell it could be deformed resulting in no protection in that region of the lid!!
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otherwise I'd still be using it. 
Again, magazines get paid to say thou shalt replace every 5 years with thine tyres 
