It is proper Netiquette to sign messages. Do include a signature at the bottom.
An electronic signature is an electronic depiction of your name with contact information to back it up. For a full explanation of an email signature see that section.
Use a signature to make an email message valid because a signature is the authors stamp of approval. Email messages without signatures are informal. They are personal messages between close friends. If the message is important sign it.
Sign messages because it gives them credibility.