E-Mail Header: MIME-Version: 1.0 - When should this be used?

This could be rightfully construed as a beginner's question.

Consider the following e-mail header:

MIME-Version: 1.0

My Questions:

• When should you stamp an e-mail with this header?
• When should you omit this e-mail header?

EDIT: Is it true that whenever you specify a 'Content Type' header, no matter what that content type happens to be, that you should also specify 'MIME-Version'? [This link tipped me off to the possibility.]