icraig :
I disagree, the whole point of TIM is to make the thermal contact between the CPU and heatsink better. It doesn't go 'off' nor does it dry out.
Hell I've got a PC built in 1998 sat in front of me now still working with its original TIM, and it still works.
And I can bet a heck of a lot of PCs in the world used by 'normal' users don't ever think about it, and have no problems.
I say if theres no issues, don't bother!
I do agree with "If it aint broke, dont fix it".
But this topic is heavily dependent on the compound in question.
For example, many thermal compounds use a dissolved silver compound for its great conductive properties. BUT, Silver does tarnish (oxidize) and will loose its conductivity over time.
On the other hand there are silicone based thermal compounds that will separate and migrate over time and cause air gaps between components.
There are many compounds that can be used for a permanent solution and do not migrate or oxidize and many OEMs use these to ensure proper longer term functionality.
Although as i said before, most "enthusiast" marketed compounds are geared towards performance not longevity.
icraig, I do agree to your stance don't get me wrong. Bottom line, if you want to be sure you are getting the best cooling performance... go ahead and replace it. But if your not having any issues and don't feel inclined, no harm no foul.