Well, that depends. The Importance flag is now commonly used by spammers to get you to open their messages. If you will be using these in your organization, please get agreement on how they are to be used. If everything is “Important,” nothing is!
On the topic of Read Receipts, these are far too easy to override. Users can either not send the receipt or not actually read the email and still generate a read receipt. Just like those Importance flags, agree as a team on when they’ll be used and what they mean.