I’ll often receive emails asking me about specific rules of word usage. Or when I submit copy to a client, one of the reviewers will send it back with a comment asking whether I know the rule about something.
The grammarians in the crowd will hate me for saying it, but there really aren’t any rules about writing. There are simply preferences that the people who control the writing expect to see.
As someone who has written for literally hundreds of organizations, I can attest that each of those organizations has its own writing styles and quirks. A construction that’s perfectly acceptable to one would be viewed as heresy by another.
Some companies go so far as to publish their own style manuals. Others point to Associated Press or MLA as their chosen set of rules — but those things are just that: a choice. A preference. So no matter how strongly you feel about the value of the serial comma (also known as the Harvard comma in some circles), know that the company next door may simply abhor it. And you’re both right.