Melissa, if your lower teeth show, it may simply be because your lower lip has sagged a bit; there is a lip lift for the lower lip too.
My lower teeth have never showed. But, I have a very acute dental arc, even when I smile very hard, only 6 teeth show. I've got a congenitally underdevelloped jaw. Basically, my dental arc, chin height and upper and lower jaw bone and gonial angle is average for a female. Not sure about the teeth, but I wouldn't be surprised if they were also in female range (I don't quite know where to get that data). Only my masseter is in male range.
One thing I've found interesting is that low testosterone in the womb has an effect on the digit ratio and certain facial caracteristics like nose breadth, jaw and chin. A lot of the head caracteristics are devellopped even prior to puberty, and even birth. On average, the male childs with the most gracile heads pre-puberty are those that have the lowest T exposure according to the digit ratio and they keep those features into adulthood.