I generally encourage designers to complete their centerpiece by filling in the back of the design with leather leaf fern. Some florists skip this step, reasoning that it saves on flower costs.
But filling in this hole doesn't take that much time or greenery. I always advocate that the bride should arrange (if possible) to have her church florals transported to the reception venue and use as a display there. Since it may be viewed from all sides, it's faster and looks more professional if all sides are finished with no foam showing.
Cut your leather leaf into shorter pieces, quickly filling in the design enough to cover the foam and tape.
The polished look is more pleasing - even if it's not going to be viewed from the seating in the church.
Your basic greening is completed and could be flowered in now. I do like to add in more different types of greenery, because the variety of textures and colors add as much to the design as the flowers do.
It's your choice from here whether to add in the additional varieties or to simply choose to begin to flower at this point.