Use Reply All
In general, all the time. What? That's crazy, right? Nope. Someone crafted the addressees in the email you are reading for a reason, and respect that. I'm referring, of course, to typical email threads with a small group of people -- there are exceptions, and I'll get to those in a moment. But if you click reply to a mail with a bunch of addressees on it, you identify yourself as either clumsy and thoughtless or someone who doesn't respect the people on the CC line enough to include them in the conversation. Which of those would you like to be known as?
When an email has an extremely wide distribution -- such as to an entire division. Don't ever reply all to a request for information from an admin, for example. You'll annoy hundreds of people and probably start a firestorm of "stop replying all" messages that will only make matters worse.
When you need to narrow the focus of the conversation. If there are a half-dozen people on the thread and you want to branch the subject or interject something confidential or sensitive, be highly aware of whom you are cc'ing.
Don't Use Either
If you're inclined to make a joke at someone's expense, say something impolitic, or be otherwise insensitive, don't. I've heard advice like "be sure to narrow the audience if you're going to say something at the expense of someone cc'd on the mail," but that is just dumb. Remember that email lives forever, and it can be forwarded -- accidentally or intentionally -- without your permission at any time. Don't risk it.
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