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currently headers are only re-encoded only if header
contains symbols other than 

   -a-zA-Z0-9!*+/[]., 

So latin-only users shouldn't really worry about
re-encoding. All others are most probably use mailers with
support of rfc2822 header encoding (there is no other valid
way to put native characters to headers except rfc2822
encoding). 

I can't see any problems with that.. Anyway could you
provide any usecase where current bahavior can couse problems?