I've snooped around and found a in the coding of the editpost template, the editpost-inc-php and other modules.
But I can't be sure any of them will remove the deault "CHECK"
One of the modules had this code:
if ( $user['USER_ID'] == $Postedby ) {
$markeditselect = "<input type=\"checkbox\" value=\"1\" name=\"markedit\" checked=\"checked\" class=\"form-checkbox\" /> {$ubbt_lang['MARK_EDIT']}<br />";
I think the checked=\"checked\" does it but only on the "if condition" is true.

Any other ideas???