If you're not overclocking then you needn't replace the stock HSF unit unless you have particular issues with your temperatures due to your location, ambient temps etc. Keep an eye on your temps and see how they hold up, but as long as they are within comfortable limits at idle and full load I wouldn't worry about replacing the stock HSF. If you do overclock at any point then obviously it's a different story.
HWMonitor from CPUID is a good utility for monitoring all of your hardware and will also show you the temperatures of most of your components:http://www.cpuid.com/hwmonitor.php
Hope this helps and good luck!