Should I edit a cost rule or add a new one?

Add a new one whenever the cost actually changed, and edit only when the old rule was wrong. Saving a new cost with an effective-from date closes the previous rule the day before and opens a new one, so your history keeps the value that was true then and only the days from the new date forward are re-priced.

Editing a rule rewrites what was true. That is right for a typo and wrong for a price rise: a supplier increase entered as an edit makes every order you ever shipped more expensive, and last quarter's margins change while you are looking at them.