Hmm looked into this again. The "Show your Signature" on new post/thread, should always be on. The only way i can think of that would cause this, would be some hack/modification.
search for the following code in newreply.php and/or newthread.php:
PHP Code:
if ($bbuserinfo['userid'] AND !$postpreview)
{
// signature
if ($bbuserinfo['signature'] != '')
{
$checked['signature'] = HTML_CHECKED;
}
else
{
$checked['signature'] = '';
}
}