"Many employee communicators use the Associated Press Stylebook for their internal publications and other written content. To many, AP is “the bible” and its rules must be adhered to strictly. But this intractable approach has some major flaws.
What AP does well is bring consistency to writing. I believe organizations should use AP as a starting point and create their own supplemental internal stylebooks. I give the following advice: It’s OK to contradict AP as long as the deviation is (1) well thought-out and (2) applied consistently."
As this article points out, AP and other common style guides were created for another purpose. And even those target audiences make exceptions.
Pick the one that works for you and keep track of your exceptions. Share them by posting to your intranet or wiki and encourage everyone to use the same format for all purposes (newsletters, blog posts, etc). The key to a successful style guide is consistency.