I usually copied my boss on most email. It was mostly a CYA move and always came in handy when someone was looking for someone to blame for their incompetence. Last thing any supervisor needs is someone from another department dropping a hot steamer in his lap and blaming one of his guys.
Thing was, reply to all was the norm, and quite often the distribution list would get quite large. Taught me how to be diplomatic and still call out lazy or incompetent employees.