for my version of UBB, 7.4.1, the changes had to be made in the scripts/newpost.inc.php and scripts/newreply.inc.php files. the template files, editable from the control panel, just contained smarty code for $addtofav.
i just looked for addtofav in the inc.php files, then added the checked='checked' code to the input tag.
works a treat!