The stock cooler is designed to keep the cpu within the specified temperature limits set by the cpu maker.
No NEED for a new one, but it would give you better temps and most times more quiet operation.
If your case has good air flow, stock is just fine. Think of how many cpu's would fail if the stock cooler was not upto the task?
Users with aftermarket cooling makes up a small part of the total number of users.
My only warning is to ensure you check the cooler for dust build up every few months to ensure you do not get dust build up leading to overheating.