When a citation includes a digital object identifier (DOI; see Electronic Sources and Locator Information [PDF]), no further retrieval information is needed.
When a DOI is not available, and a URL is included, do not include retrieval dates unless the source material may change over time (e.g., wikis).
via www.apastyle.org
Official APA style for retrieval dates. I'm not sure I like the second part of this. It's not only wikis that are likely to change; any website could disappear or restructure its URLs.