Proper punctuation

Question: When you have a list of items separated by commas, should you put a comma between the last two items, even though they're already separated by the word "and"?

In other words, should you write "Item one, item two, and item three" or "Item one, item two and item three"?

One sentence I just read which illustrates the need for that last comma is,

I want to thank my parents, Ayn Rand and God."

The Gregg Reference Manual agrees that the comma before the word "and" is necessary. It seems there was a court case where the division of an estate hinged upon that comma. The will stated the estate was to be divided equally among child one, child two and child three."

The judge ruled that the commas divided the object of the sentence into boxes, and each box would receive half the inheritance. Child one got half, and child two and child three got to split the remaining half between them.

Someone like Judge Ooka might have checked whether that was what the deceased had intended by stationing people at his grave to listen for the sound of someone rolling over at the moment the judgment was read.