unless it's in danger of overheating, why bother? you wont increase the lifetime of the cpu by a single day, it wont perform a single percent faster.
It's worth getting quality paste when installing the heatsink anyway, but it's not worth replacing it just for the sake of replacing the thermal material.
If it's a new CPU, I probably wouldnt bother unless you really skimped out on the cooling... in which case the price of AS5 would probably be better spent on a case fan. Of course, if you're overclockign heavily or going for an uber quiet PC, you probably would already know the answer to be the other one, and would be using a after market HSF
