Conditional addition of css - hook_init?
If that CSS is in your module, then the module should really be implementing the hook. 

I don't think you can do anything with CSS and JS in a hook_html_head_alter(); that is really to mess with the render array before it is turned into $head by template_process_html().  Personally, I would use a template_preprocess_html().  You can add your logic there.  For example, 

function mymodule_preprocess_html(&amp;$variables) {
  if (drupal_is_front_page()) {
    drupal_add_css(drupal_get_path('module', 'mymodule') . '/mymodule.css');
  }
}