I guess it should be parsed "everywhere" (i.e. also in the mini-profile next to the posts), in that case, the best place may be Load.php, in particular loadMemberContext:
'title' => !empty($modSettings['titlesEnable']) ? $profile['usertitle'] : '',
'blurb' => $profile['personal_text'],
to:
'title' => !empty($modSettings['titlesEnable']) ? parse_ccb($profile['usertitle']) : '',
'blurb' => parse_bbc($profile['personal_text']),
emanuele is not 100% sure the lines are those, but they look pretty much what I would expect them to look like...