I don’t think there is anything more annoying than trying to figure out if a comma is needed or not. There are so many rules, and they don’t necessarily agree. One editor likes the Oxford comma and another hates it.
I tend to use the Oxford comma, where you place a comma after the ‘and’ when listing a number of objects. Thus: He ate a sandwich made of ham, lettuce, mayonnaise, and cheese.
Some of you will say the last comma isn’t necessary. You may be right. It doesn’t add anything. The ‘and’ separates the ingredients.
But what about this sentence? She enjoys everything about Christmas including candlelight service, wrapping presents, caroling and decorating the tree.
Does she sing while decorating the tree? Maybe she does. I mostly grumble under my breath because the lights are tangled up, but that’s another story.
I think a comma lets the reader know that while the items are connected, the last two in the list are not married to each other.
My rule of thumb is to be consistent. Either use the Oxford comma throughout, or let ‘and’ go unaccompanied.
If things get confusing, you can always make a exception (remember, there’s an exception to every rule) and pop that comma in.
After all, the object is clarity. And I guess that should be the only ‘rule.’