Uncheck "Send the new user an email" by default

There’s no filter or anything, but you could do it by adding some Javascript to the new user page:

function my_disable_new_user_notification_default() {
	echo '<script>var field = document.getElementById("send_user_notification"); field.checked = false;</script>';
}
add_action( 'user_new_form', 'my_disable_new_user_notification_default' );

A bit hack-y, and I’m generally loath to use Javascript when it’s not necessary, but it works and I can’t see another way.

Perfect!
Thanks heaps!