If you see the header
Foorbar: XXX (=?iso8859-1?q?=A3-=A3?=) YYY
then if it is a structured header you are supposed to decode the
encoded-word, and if it is an unstructured header you are not supposed to
decode it.
If you don't know whether it is structured or unstructured then you are,
officially, stuck.
but it only matters for the purpose of display. if you're using an encoded-word
for anything other than purely human-readable text, you're not using them
properly.
so for instance use of encoded-words for newsgroup names is an improper use.
so if an implementation doesn't recognize a field and treats it as unstructured,
the worst that will happen is that it won't decode some encoded words before
display (because it won't recognize them next to some 'special' characters,
or it won't recognize that parentheses are comment delimeters).
and outside of to/cc/from/bcc/reply-to, few structured fields contain human
readable text anyway.
Keith