Board Thread:The Panopticon/@comment-55646-20130411174713/@comment-1432718-20130411181526

There was a discussion, long ago, that resulted in the policy Tardis:Doctors. The terms actually have been used on screen (TV: The Three Doctors for example), and it was decided that they are always capitalized.

If you're interested, I can try to hunt up the original discussion, but the policy page should answer your questions.