My company has no direct policy about this and we generally have an informal way of communicating, especially within teams. Between teams, I have seen varying behaviors in the past 12 months I am here - from only writing directly, to CC'ing the entire department, or even superiors - in all cases people who are completely unrelated to the subject of the e-mail.
Being both on the receiving end and sending end of e-mails, I generally find it uncomfortable to CC people who are not related (or when people writing to me CC others in my team e.g. my boss or the entire department), as it seems to signal lack of trust. Generally, it also makes the e-mails less personal, which I feel doesn't help if we work in an informal environment where bonding and open communication is imperative to teamwork.
My current practice is that if it is somehow indirectly related to others, I CC' the whole team, but when one person replies, I write all follow up e-mails only to that person.
This all goes down to building a relationship with the person I am working with, making a joke or two, communicating one to one.
In what kind of situations is it critical to CC people even if they are not related? When is it acceptable not to CC the team or the supervisor of someone I am writing to?
Since we don't have a policy on this, let me know what would be most professional but also instrumental to building good communication.