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Interesting opposing views.
When a vehicle manufacturer states a specification such as valve adjustment, that spec IMO should be followed. Even though many of us have lots of experience, we still do not possess the designers knowledge of how he wanted that engine to perform. He had considerations of wear, function, life expectancy, and yes noise that might irritate the customer in his original design. To him it is a balancing act trying to achieve all the stated manufacturer goals.
I have yet to see any manufacturer state valve adjustments to be performed on warm or hot engines. Years ago, we adjusted valves while the engine was operating, but still cold or cool. Thus the tolerance for heat expansion was considered in the spec.
When the manufacturers went to hydraulic lifters, this was done to not only control noise but to increase engine life and reduce wear. And of course maintenance.
As rks points out in his example, each valve would be opening at 100 times per second @ 6k and if the tolerances are off either way damage can be the result. As shadowfire points out too loose and the valves seat harder via the spring pullback, this can damage valves & seats. The damage may not be noticeable until it gets really bad. Also you lose a little opening time, thus reduces A/F charge into the cylinder.
If too tight then the valve may remain open enough to burn or stretch resulting in engine damage.
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