Quoting Moshe Weitzman <weitzman@tejasa.com>
In drupal5, you should call hook_node_submit() and hook_nodeapi('submit') before node_save(). that will assure that any group info gets saved (for example).
In d6 this is not needed since those submit hooks became hook_nodeapi(pre_save) and that happens from within node_save().
drupal_execute is also a valid way. the node_load() is a little expensive but this is a very clean way to proceed. if it is working for you, i would proceed like this.
The documentation to convert hook_submit to drupal 6 says to add $form['#submit'], not to create a nodeapi(presave). I'm a bit confused here. Should I convert my hook_submits to nodeapi(presave) or should I simple add $form['#submit'] to the form? I believe if I change it to nodeapi(presave) then I won't need to call hook_node_submit() before calling node_save() as this would be taken care by the presave call in node_save(). But it seems that even in drupal 6, if I use $form['#submit'] I'll still have to do a call to the hook_node_submit() before calling node_save(). Am I missing something here?