If you are not going to be overclocking, in 99% of most cases, the stock cpu cooler is fine.
However, as others have mentioned, there are several reasons why you my want to go with an aftermarket cooler:
1. An aftermarket cooler will keep your cpu temperature(s) lower than the stock cooler would
2. An aftermarket cooler will run quieter than the stock cooler in most situations (although in some situations you might need to replace the fan(s) that come with it.
3. If you have a side window that allows you to see within yor case, an aftermarket cooler just looks better than the stock cooler.
Things to research before purchasing an aftermarket cooler:
1. Is it compatible with your cpu socket type? Most, but nt all, are fairly universal, meaning they will fit most socket types. But check to make sure.
2. Is it compatible with your motherboard? Make sure it will not interfere with any motherboard connections such as power connectors, RAM slots, PCI-E slots, or any other otherboard hook-ups.
3. Is it compatible with your case? Different cases will allow for different heighth cpu coolers, make sure yours will fit!