You may or may not use a serial comma wherever (or nowhere) else you want, but woe to the person who leaves the serial comma out of legal documents, including contracts, legal opinions, statutes, regulations, and any other legal agreement or law that will be interpreted literally (and I mean literally, literally) by parties to the agreement or by a judge – or even, mercy, by a legislator or government lawyer.
Honestly now: Do you really want to pay lawyer(s!) bills and court costs when lawyers, judges, and clients end up doing battle over the meaning of a sentence when the presence of a comma would have allowed everyone to go home to supper and a peaceful night’s sleep?
Lawyers, legislators, drafters of laws, and judges have enough trouble writing clearly without adding to their own and their readers’ woes. Use the serial comma, please. It will save all of us time and money.
May I also give a plug for the delightful: “Dreyer’s English: An Utterly Correct Guide to Clarity and Style,” by Benjamin Dreyer, who also writes about serial comma use, although not in the context of legal documents.