If I get an email from a head manager in the company, and other managers are referenced in the CC section, should I respond to everyone? Or do I add those other emails in the BCC/CC spots too?
It would depend solely on the content of the email and the response. Sometimes you want to keep everyone in the loop and sometimes you want to provide some private information.
For instance, Sometimes I get emails with a long list of CCs that include clients. I may want to discuss something with the Project manager without the client seeing it, so we can honestly talk about the potential problems so then I would I would remove them from the list and let the PM figure out how to give them the news once we have figured out what our position will be. Other times, especially when I know the issue is going to be important to all the CCs on the list, I keep everyone on it. And sometimes, people are cc'd who really won't care about any responses. For instance, we get some emails that are sent to just about everyone but really only a few people care if I need to tell the DBA that Thursday is not a good day to refresh the QA database.
Especially avoid CCs if your reply is something like Thanks with no other detail. 37 other people don't need to know that you were polite. Another one is when the email is something like "Joe is having a birthday today". Don't wish Joe and 45 other people Happy Birthday, it is fine to respond just to Joe in this case.