Why do you say that? I don't understand why lower jaw expansion can't be good. But it's true that if I protrude my lower jaw (imagining my upper teeth are already expanded forward), my jawline looks good enough, so maybe I don't need it.
Update: I'm actually not sure by what you mean with "look like the first slice of bread". Are you saying it because I could get a square face and jaw like Dr. Mike Mew? If that is the case that is what I'm looking for. I would also expand slowly until I see a good enough result.
http://imgur.com/a/E5FJ4Here is an example of what I mean and I believe that the triangle jaw is much more aesthetic, that is why I was thinking of starting a topic some time ago if lower jaw expansion is really better and worth it. I am no specialist, nor do I have a lot of knowledge about stuff like this, so dont take my word too seriously. I just know what your goals are and wanted to throw this is in your favour, nothing more, nor less.