The importance of a sign-off depends on the context in which you're sending an email. If you're sending a memo via email to a co-worker or some class notes to a classmate, it probably isn't such a big deal. However, the wrong email sign-off could send the wrong message. For example, if you send 'Yeah, thanks' to someone who is trying to help you, it could come across as snide or sarcastic. Unfortunately, text-based communications lack context, which means it's easy for your recipient to get the wrong idea. So, beware of how your reader will take the message you are sending because, yes, your reader will likely see your sign-off.