I've got an overclocked 970 and it doesn't seem to need much. I've got a few case fans in my computer (two in the front, one larger in the back) and that seems to keep everything pretty cool, or more so I've never really noticed anything out of the ordinary. If you're not sure you can always just keep an eye on its temperature. Anything under 95C is under the official threshold. I have the windforce 970, which has a few fans so things can differ for your version.