As a rule of thumb I don't date content that is evergreen because the date is irrelevant to the content, however, if the blog contains time sensitive material such as news articles then the time-stamp becomes that much relevant.
Keep in mind that issues may occur if you deem your content "evergreen" when its actually not. For instance, you may publish a Photoshop tutorial which you think is evergreen, however; in order to keep it current, you have to update this content piece every time Adobe comes up with a new version of Photoshop. If that's the case then your content is not truly "evergreen." The same applies to Wordpress tutorials and such.