I remember watching a video of who I'm quite sure was Fred Bear. He was talking about the evolution of the bows his company made, and he talked briefly about applying fiberglass to both the belly and the back. He had found that adding it to the back was productive, but that adding it to the belly was not. You can't go wrong as a rookie glass bowyer following the advice of Fred Bear. I wouldn't bother with the belly if I were you.