In nearly all my writing classes, I hear the question, "Should you put a comma before and in a series?" The answer is this: It's a matter of preference. Let's see how the issue is handled by two highly respected writer's references, The Associated Press Stylebook and Libel Manual and The Business Writer's Handbook.
The Associated Press Stylebook and Libel Manual (sixth trade edition), edited by Norm Goldstein, gives the serial comma a NO vote. Here is what it says:
Use commas to separate elements in a series, but do not put a comma before the final conjunction in a simple series: The flag is red, white and blue. He would nominate Tom, Dick or Harry.
Put a comma before the concluding conjunction in a series, however, if an integral element of the series requires a conjunction: I had orange juice, toast, and ham and eggs for breakfast.
The Business Writer's Handbook (seventh edition), by Gerald J. Alred, Charles T. Brusaw, and Walter E. Oliu, votes YES for the serial comma. Let's see what it says:
Although the comma before the last item in a series is sometimes omitted, it is generally clearer to include it. The ambiguity that may result from omitting the comma is illustrated in the following sentence.
AMBIGUOUS: Random House, Bantam, Doubleday and Dell were individual publishing companies. [Does "Doubleday and Dell" refer to one company or two?]
CLEAR: Random House, Bantam, Doubleday, and Dell were individual publishing companies.
What's my take on the serial comma? I use it, but most of my clients do not. The choice is yours—especially if you pay the invoice!