Exporting settings in a code with Features / Strongarm - how to detect right variables - config ones
Unfortunately, the Drupal variables table doesn't provide any distinction between variables used for configuration (e.g., whether anonymous pages should be cached) and variables used to store transient values (e.g., date cron was last run, which tabs are currently active in the UI), so there's no automatic way to do this.  (Fortunately, it looks like Drupal 8's configuration management system will handle this better.)

I typically export and update Features modules from the command line using Drush.  I've been using the following Unix shell script to list Features Components which haven't yet been exported to any feature — the script filters out Features Components I know I don't want in my Features modules:

drush fc % --not-exported \
    | grep -v '^ dependencies:' \
    | grep -v '^ user_permission:access devel information' \
    | grep -v '^ user_permission:execute php code' \
    | grep -v '^ user_permission:switch users' \
    | grep -v '^ uuid_node:' \
    | grep -v '^ variable:additional_settings__active_tab_' \
    | grep -v '^ variable:cron_key' \
    | grep -v '^ variable:cron_last' \
    | grep -v '^ variable:ctools_last_cron' \
    | grep -v '^ variable:date_api_version' \
    | grep -v '^ variable:dev_mem' \
    | grep -v '^ variable:dev_timer' \
    | grep -v '^ variable:devel_api_url' \
    | grep -v '^ variable:devel_error_handlers' \
    | grep -v '^ variable:devel_execution' \
    | grep -v '^ variable:devel_krumo_skin' \
    | grep -v '^ variable:devel_page_alter' \
    | grep -v '^ variable:devel_query_display' \
    | grep -v '^ variable:devel_query_sort' \
    | grep -v '^ variable:devel_raw_names' \
    | grep -v '^ variable:devel_rebuild_theme_registry' \
    | grep -v '^ variable:devel_redirect_page' \
    | grep -v '^ variable:devel_use_uncompressed_jquery' \
    | grep -v '^ variable:devel_xhprof_directory' \
    | grep -v '^ variable:devel_xhprof_enabled' \
    | grep -v '^ variable:devel_xhprof_url' \
    | grep -v '^ variable:drupal_css_cache_files' \
    | grep -v '^ variable:drupal_http_request_fails' \
    | grep -v '^ variable:drupal_js_cache_files' \
    | grep -v '^ variable:drupal_private_key' \
    | grep -v '^ variable:entityreference:base-tables' \
    | grep -v '^ variable:features_codecache' \
    | grep -v '^ variable:features_ignored_orphans' \
    | grep -v '^ variable:features_semaphore' \
    | grep -v '^ variable:install_profile' \
    | grep -v '^ variable:install_task' \
    | grep -v '^ variable:install_time' \
    | grep -v '^ variable:mimemail_key' \
    | grep -v '^ variable:prod_check_module_list_lastrun' \
    | grep -v '^ variable:simplenews_last_cron' \
    | grep -v '^ variable:simplenews_last_sent' \
    | grep -v '^ variable:simplenews_private_key' \
    | grep -v '^ variable:simplenews_source_cache' \
    | grep -v '^ variable:update_last_check' \
    | grep -v '^ variable:xmlsitemap_generated_last' \
    | grep -v '^ variable:xmlsitemap_rebuild_needed' \
    | grep -v '^ variable:xmlsitemap_regenerate_needed'