There's an interesting car commercial running now. It states that "Joan of Arc reigned only five years." Really? I was unaware she ever reigned. As a matter of fact, Joan's fondest desire was to put someone else on the throne. There's no doubt that Joan had a significant influence on the course of history, but she wasn't a sovereign, and she didn't reign.
Recently, I saw a similar use of this verb in the context of a college president's "reign" over the campus. I know academic regalia is cool, but it generally doesn't confer royal power. That's why college presidents are installed during an inauguration rather than a coronation.
English is a language of subtleties, and, to me, "reign" as a verb has the connotation of kingly (or queenly) rule. As an adjective, though, "reigning" often has the opposite connotation, meaning commonplace or popular, as in the "reigning" opinion or fashion of a particular time.
One more thing to remember is the spelling of reign v. rein. A "rein" is part of a bridle, used to hold a horse in check. However, I can't tell you how many times in the past few weeks I've seen the phrase "to reign him/her/it in." The right word in that context is "rein" because it means to exercise control over something or someone.
And don't forget to take your umbrella when it reigns.