To be fair, rfc2047-style headers are fully backward compatible. My procmail installations work through a lot of mail including theses header extensions without any problems.
Special handling is only needed if you want to modify and/or extract the mail headers via a natural format. E.g. the result of
$ formail -I "Subject: i am naïve"
will not be compliant with rfc2047, and
$ formail -x Subject
will *not* undo any encoding and just return the raw header content. Not a big deal, I think.