The only thing that you ever have to worry about with a normally clocked, normally volted CPU, that never gets a power surge or a meteorite strike (
) is something called "hole depletion". Running a CPU at maximum speed 24/7 will eventually cause it to fail, or become unreliable, but so will running it at minimum speed 24/7.
It's basically just electronic fatigue, and it affects almost every piece of electronics. Sooner or later, the thing will just wear out from use. The good thing is, if nothing bad ever happens to the computer, the CPU should wear out long after the whole thing is obsolete.