When someone sends you an email and Cc’s other people, you should reply all.

Is your information really relevant to everyone you’re replying to? That’s the question you need to answer before replying all. You don’t want to be jamming people’s inboxes with unnecessary messages. Watch minutes 0:37 to 0:58 to listen to what Adriana Girdler has to say about this: https://youtu.be/oI3rVQFye9w?t=37