If you're not a hardware enthusiast into overclocking and benchmarking I don't think it is really necessary to buy thermal paste and re-apply it. With the stock intel/AMD heatsink you'll be lucky if you can get a 5?C change in temperatures. Not enough to impact performance or lifespan of your processor. Stock thermal compound on most heatsinks, while not spectacular, is perfectly adequate. However, after a few years when your thermal paste begins to degrade or dry out you may consider re-applying it. I would buy thermal paste then rather than now.
If you enjoy overclocking and benchmarking buy an upgraded heatsink along with thermal paste to make a worthwhile impact on temperature.