I think it's amazing beautiful when a person's chin curves outwards, unfortunately my chin is not as curvy as I would like it to be. A little shadow under my lips gives an illusion that my chin curves up a little. However in reality my chin curves slightly down. ^.~ That is why I try to avoid side profile shots. This is also important for people who have an little extra skin under their chin.
Blending is important, in different lighting your shadow will look darker, make sure you blend well so it won't be noticeable. I find this makeup trick helps bring back youth and a hint of cuteness. Try it next time and see if you notice the difference. <3