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Ticking is *not* good for the valve. RUN AWAY from anyone that says this. Let me explain why.
As the cam shaft rotates and the cam lobe approaches the valve, ideally it starts picking up the lobe as soon as the lobe starts changing shape. The lobes are profiled so that the initial rate of lift is slow, then it accelerates to fully open, slows, starts closing, accelerates towards closed, then slows before completely closing.
You hear the ticking sound because the valves are too loose. If the adjustment is too loose, you end up skipping the gradual speed change at the beginning, and end, of the valve cycle. There is now no initial slow lift, and the cam lobe basically slams into the valve (faster than intentioned). Additionally, there is no deceleration at the end of the cycle, and the valve slams shut against the cylinder head. This puts a ton of stress on the valves, especially at high rpms. It is not good for the longevity of the engine, and could result in catastrophic failure if the valve stems bend (due to getting whacked at the start of the cycle) and seize the valve, or damage of the valve due to slamming back into the cylinder head.
Conversely, if the valve adjustment is too tight, the valves don't come to a complete close, and you lose compression (and therefore, power).
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