Do I need to use a block or form to implement login box, or can I just post to a 'login' url the user details?
You can print login block in the tpl file. No need to use username/password fields of HTML/CSS.

&lt;?php print drupal_get_form('user_login_block'); ?&gt;


For Registration :

&lt;?php print drupal_get_form('user_register_form'); ?&gt;