Why YSK?
The first person who typed “should of” probably heard of it in real life that was meant to be “should’ve”, they typed “should of” online and readers thought that it’s grammatically correct to say “should of” which is in fact wrong and it became widespread throughout the years on Reddit.
I hope something could start to change.
It’s mandatory in a series, only. Something is only a series of there are three. Plenty of time the cadence and diction sounds like a series but isn’t.
If the first two or last two are antecedent to one another, you don’t need the comma. Said another way, if the first or last noun is not severed from the second, you need a serial command to indicate that.
It depends on what you’re trying to say.