I have one canonical web site, http://example.com. I also got the .net and .org, domains, so I have 301 redirected all the following to my canonical web site:
http://www.example.com http://example.net, http://www.example.net http://example.org, http://www.example.org
In Google Webmaster Tools, I specified http://example.com as the preferred domain.
All of the above works fine. Now the question:
The instructions for Google Webmaster Tools implies that I should register all six sites with Google Webmaster Tools. I would think that registering the one canonical web site is adequate. Which is it?