The general rule is that one style or the other should be used consistently. However, the last comma can serve to resolve ambiguity, particularly when any of the items are compound terms joined by a conjunction, and it is sometimes helpful to the reader to use an isolated serial comma for clarification even when the convention has not been adopted in the rest of the text. In

cider, real ales, meat and vegetable pies, and sandwiches

the absence of a comma after pies would imply something unintended about the sandwiches.