The great "Reply" versus "Reply All" debate has been going on since email started. The reason it's an issue is that people in group messages get annoyed when they receive unnecessary replies in their inbox.
Consider the following scenario: Your boss sends a message to her entire team asking for updates on a particular project. Your boss is really the only person who needs to know the answer, so in this case you should use "Reply," not "Reply All." If you use "Reply All," everyone who received the original message will get your response when they don't really need it.
The basic rule is to use "Reply All" only when your reply truly needs to be seen by the original sender and all people in the "To" and "Cc" fields. So, "Reply All" is appropriate for a group email discussion about a work project that directly involves every recipient in the group.
If only some of the recipients of the original message need your response, use "Reply" to reply to the original sender and then manually add the other necessary email addresses in the "To" field.
Remember, managing everyday email and spam is hard enough. Don't add to people's irritation by using "Reply All" when "Reply" will do.