Add an option to switch notification default setting

Hello,

To avoid unnecessary sending of email, I would like to check by default the "Do no send notification for this update"
- when creating the content
- when editing the content
- when adding some comment

I tried with :
$form['oa_notifications']['skip_notify']['#default_value'] = 1;
or
$form['oa_notifications']['skip_notify']['#attributes'] = array('checked' => 'checked');

Checkbox is checked in both cases, but notification are still sent.
If I uncheck then recheck, no notification are sent.

I think it is because of the ajax callback but I don't know how to fix it.
Any help is welcome.

[SOLUTION]
add javascript :
drupal_add_js('jQuery(document).ready( function ($) { $( "#edit-oa-notifications-skip-notify" ).trigger( "click" ); } )', 'inline');