I don't know about policy but I completely agree that as a captain, you should somehow inform the pax -directly or via the CA- of whats going on when things aren't as they should be. But then ofcourse, only if capacity permits... making minute long anouncements just prior to taxi or during the descent seems daft, but it happens frequently.
Otherwise my oppinion is: say something when things aren't as they should be but otherwise, keep quiet except for the very short announcement when established on cruise and things cool down a bit.
regards/lns