The copyright and the license are two different things.
(c)2017 Joe Blogs in a document is stating that you own the copyright of the work and are claiming the rights related to the copyright. If you don't openly display your copyright ownership then you may later need to offer proof that you created and published the work in any dispute and may even be asked to provide a compelling explanation as to why you didn't openly claim copyright at the time of publication.
As the owner of a copyright you have the ability to license the work to others. This gives you the power to choose and enforce the terms the work is licensed under.
As for usernames, "Joe Blogs" is the legal copyright owner. Authors can use pen names and have those names displayed in copyright notices so the same should apply to online usernames. Using an online name for copyright purposes could require further proof that you used that name at the time of publishing the work, which would need to include the old and new usernames to prove that both usernames were used by you.
While formal copyright registration has been voluntary for some time, it can be useful for some works such as art or photos to help prove ownership at a later time. For most works it is common practice to display
(c)2017 Joe Blogs as a claim of copyright.