Can any of the cache_* be emptied safely?
With one exception, yes. That exception is cache_form. That is not actually a cache table but contains temporary $form_state information of forms currently displayed on your site. Clearing that table would invalidate these forms and when a user then tries to submit that form, he will get an error message and has to fill out that form again.

Note that drush sql-dump and Backup and Migrate automatically leave out the content of these and other tables when creating a database dump with these instead of mysqldump or phpmyadmin.