I think that if someone were to do incline bench only for a year, his chest would look pretty much the same as if he had done flat or decline only for a year. Your chest looks the way it does genetically, not much you can do to "sculpt" it. You can make it bigger or smaller, like any other muscle, but that's it.
I think when most people talk about "building up the lower chest," what they really need to do is reduce the fat that often resides there.
I'm not saying that the various incline levels are useless at all. I do all of them myself. But only because I believe that that is part of a thorough chest workout. Not because I'm trying to hit one specific portion of the chest. It's all one muscle after all.