ahah and fieldset 'collapsed' state
I'd do something along the lines of checking the value of share_with_rep_region before defining the collapsed state of your fieldset. i.e.:

// whatever the default value of the select is before 
// the user has interacted with it in any way
$default_value = '';

// set collapse true
$collapse = TRUE;

// assuming you're passing $form_state into your form definition, if not, do so
if ($form_state['storage']['add_rep_share_fieldset']['share_with_rep_region'] !== $default_value) {
    $collapse = FALSE;
}

$form['add_rep_share_fieldset'] = array(
    '#type' =&gt; 'fieldset',
    '#title' =&gt; t('Share with Rep'),
    '#prefix' =&gt; '&lt;div id="' . $cur_div_wrapper_name . '"&gt;',
    '#suffix' =&gt; '&lt;/div&gt;',
    '#collapsible' =&gt; TRUE,
    '#collapsed' =&gt; $collapse,
);


I hope this shows the basic idea behind what I'm trying to say: $collapse = true by default, IF the user has interacted with the form in any way, $collapse = false