If in Doubt, Coordinate
I've seen a lot of hard feelings created because someone was not copied on a memo.
There is, of course, a need for balance. Most of us receive copies of items in which we have zero interest. The cheerful use of the Delete button is a partial remedy to that excess.
Over-coordination, however, is a minor irritant compared to the sin of exclusion. The excluded party carefully examines all of the included ones and asks, "Why were they copied when my operation has just as much, if not more, interest in the outcome?" The answer may not be benign. Many a conspiracy theory has been launched by a missing name.
If you sense that a person might have a sliver of a claim to being legitimately upset by an exclusion, send a copy. Who knows? You may be surprised by the intensity of the individual's interest.