I use to be a big believer in the whole 'get the best thermal paste you can' ideology, but now... Meh.
Not to contradict everyone as thermal paste's job is to facilitate the transfer of heat from the hot CPU to the cool copper of the HSF, but... When my PSU died and I had to remove my gigantic
Tuniq Tower HSF to remove and replace it, the thermal paste was all dried up and flaky.
Currently my HSF is connected directly to the CPU and held into place with 'tension screws' (long screws with big springs that keep a relatively even tension across the whole bracket) and it's cooling just as good as when it was first put on with brand new Arctic Silver thermal paste.
If you're going to be concerned about thermal paste
(which is NOT a bad thing!), then I think you should also be concerned about replacing it every few years as well.
I'll clarify my point to avoid any confusion:
By all means use thermal paste, I cannot see it doing any harm. But I will not be all that concerned with it anymore as
from my experience the HSF plays a bigger role.