You are certainly correct about (some, perhaps many) e-mail generators using tabs when folding, which is AFAICT, much more likely an incorrect implementation of RFC 2822 rather than an intentional transformation of the user's specified Subject line. Some*, however, dutifully unfold those tabs back into the Subject line, causing all sorts of strangeness -- explosion of conversations that are logically identical, except that in one, it's all spaces, and in the others, some of the spaces have, seemingly arbitrarily, turned into tabs. 

Fixing all of the e-mail generators to Do The Right Thing wasn't what I had in mind, but making sure that mailman, a rather commonly used mail service, and its reliance on python's Header class, would no longer permute the messages made my e-mail generators that do Do The Right Thing, was one step.

I would be inclined to, were I a contributor, make it so that it would conform to RFC 2822 unfolding and preserve the FWS accordingly. I don't know if there'd be enough desire for your alternative, but if there were, I'd make it a non-default option to de-tabify Subject lines (or other headers).

As it stands, I am newly freed from any restrictions from contributing, and so I might try and see what mailman's intentions are surrounding taking updates to python before trying to propose a patch that they might never use (i.e., they may just take the version you're working on now.)

*Microsoft Entourage (which I worked on while at Microsoft) and Microsoft Outlook both behave correctly, as far as I could determine in '05, with regards to the folding/unfolding headers.