To add to what Jakebeal stated, going with the date you LAST touched is important to get in the habit of from a work perspective too. If you are working on a document that helps write out the steps needed to be taken for a certain process, documenting when it was last touched is key for co-workers. This helps them to know if it has been updated or not any time recently. If it is older and they know the process has changed, they may ask for you to change it or go in and change it themselves.
Say though that you put in the date you first started the document, but, due to doing other duties, takes you 3 months to document this process. Someone may think the document is not up-to-date or missing information that may have been provided after the date. So by always keeping the date of when you last touched it current, you help others know the validity of the information.
If all else fails.... there is an option in word to allow for the date to be changed automatically to when you next open/edit I believe. I know there use to be not sure if it is still a feature.