This question is not black and white in the sense that I can give a straightforward answer to this. To tell somebody an unpleasant truth when it's very, very important whether we have permission or not, go ahead and do it, though it's unpleasant.
On the other hand if it's not so important and we just want to tell somebody some unpleasant truth although we have no permission, then we shouldn't do it. So you would have to give more details about the situation as to whether it could be okay or not okay.