Always ask your client its preference: serial commas or not? If no preference, the governing criterion for the serial comma would be, which style guide does the client prefer? If the Chicago Manual, then, serial commas OK. If AP, serial commas, not.
If there be no preferred style guide, here is the rule of thumb. If your client or employer is an American over the age of 40, do not use serial commas. American schools in the last century taught that the last element in a series should not be preceded by a comma. Otherwise, use serial commas.
The important thing here is that you — as a working proofreader — want to avoid being the ultimate authority. As any experienced proofreader already knows, second-guessing the proofreader is the blood sport of support staff. Staff would waste time challenging your discretion. You may be tempted to waste time defending it. The best tack is to refer to higher authority — especially a printed book — which obtains even more totemic authority among non-readers than it does among readers.
Online, serial commas are more than a matter of style. International readers may be accustomed to logical — rather than ornamental — punctuation; they would appreciate the serial comma. Also, online publications are read by machines — such as search engines — and machines can parse logical punctuation more easily than ornamental punctuation.
Totems are chosen arbitrarily for the sole purpose of making the physical world a comprehensive and coherent classificatory system — Claude Lévi-Strauss, author of Totemism.
Totem and Taboo is a book by Sigmund Freud
