Stock coolers are simply designed to keep a CPU in an operational temperature range at stock clocks, nothing more, nothing less. Without an overclock and with adequate ventilation, the stock cooler will work fine.
That said, most aftermarket coolers are better in a great many ways and I would suggest getting something if you intend to overclock by any significant margin or are using a very airflow-limited form factor. While I personally use Corsair closed-loop coolers, a Kraken would be fine as well.