How do I centre all page titles site wide without centring post titles or site title?

Perhaps something like

$the_post_type = get_post_type( $post->ID );

switch ( $the_post_type ) {
    case 'page':
        // you'll need something else to distinguish "landing" pages from normal ones
        // which would allow you to then set a flag for it 

        if ( $some_condition ) $landing_page = true; 

        if ( $landing_page) {
            // display your title with the class that'll centre it
        }
    break;

    case 'post':
    case default:
        // display your title with the normal class
}