I would assume that the use of "an" before words with an asperated "h" is merely a holdover from English pronunciation, which more consistently drops the "h" sound. If you are not using English spelling in your writing (e.g. "colour"), I should think you should not use "an" before historic/al/an etc. It is not a matter of "correct" English, but simply "preferred" style and consistency. "Correct" style changes constantly, especially in America, where we are less concerned with maintaining tradition. We have used the AA guide for years, but it has no more claim to correctness than the SHA guide or Chicago Manual. My wife is an editor and she frequently tells me that what I learned in 8th grade grammar comp is incorrect. But she works at a newspaper, which used the AP stylebook. So, when the Great Editor (whoever [whomever?] happens to be editing my work) tells me that's how it's done, that's how I do it (although I might grumble). Now when the editor tells me to change the sentence to say something I don't mean? that's when I fight.

Jeff Holland ("An 'istorian")

A foolish consistency is the hobgoblin of little minds, 
adored by little statesman and philosophers and divines.
--Emerson
btw, if you haven't found this site, it is wonderful for pithy quotes from people who are better writiers than you: http://www.quotablequotes.net/