We've all been on the receiving end of email messages that have a vague or content-free subject line ("Hi" for example) or even no subject line, so it's wise to remember it's always good netiquette to add a meaningful subject line to email messages.
A good subject line will help you and the recipient find your message again. So help your readers (and yourself) by summarizing your message in the subject line.
For concrete examples, read this 1-page article, How to Write a Good Email Subject.
And when a message is very short ("Cake in the break room for everybody" for example) it can fit an email's subject header perfectly. If your subject line contains the entire message, consider writing <EOM> at the end of the line so people know it's the End-Of-Message, and the need not open the message.