I've never replaced the heat sink thermal paste unless I am changing the cooler. If you do want to change it anyways you will probably see a 2C temperature change by using a decent thermal paste. Keep in mind you would have to clean the old thermal paste off the heatsink and the CPU.
Unless you are overclocking it I wouldn't bother.
However if you are going to remove the heatsink for cleaning then you will need some more paste anyway.
I've never replaced the heat sink thermal paste unless I am changing the cooler. If you do want to change it anyways you will probably see a 2C temperature change by using a decent thermal paste. Keep in mind you would have to clean the old thermal paste off the heatsink and the CPU.