No overclock is guaranteed to be safe. Most likely nothing bad would happen with a 20% OC, but who knows... let's say 80% of the processors start dying 2 years down the road... yours may only make it 1 year out. But then again they could last 10 years on average and yours would only make it 9. In the computing world I've had some overclocked processors last 5 years or more and others died after a year or two. Just push it as far as you can and roll the bones
Nothing lasts forever, but the engineers clocked the processor the way they did in order to prolong its life. Theoretically underclocking and undervolting would extend it even farther, which is why its good to have a kernel that will automatically scale the speed and voltage according to demand.
