I've got a category structure that's 5 levels deep. when coming back the groups for any publish, these groups aren't in almost any particular order. I have to discover the last child category for any publish.

If you work with them informed, you could attempt this:

<?php the_category("", "single"); ?> 

Ref: the_category

===== Up-to-date =====

Outdoors the loop:

<?php the_category("", "single", $post->ID); ?> 

You might like to try wp_get_object_terms

$categories = get_the_category();
$last_category = $categories[0];

foreach($categories as $i => $category)
{
    if($category->parent == $last_category->cat_ID)
    {
        $last_category = $category;
    }
}