It doesn't take much pittosporum in your arrangement to add an interesting color to your design.
The rounded leaves edged with white gives a nice contrast against the other solid green of the other varieties of greenery.
Cut into shorter pieces, tuck it deeply in among the other greens to help close the leaves and conceal the cut end.
Greenery alone makes a lovely cool design for churches and receptions. Many chic weddings are foregoing the blooms and settling for a large assortment of greens instead.
Greening correctly is important to learn and will go a long way in making your arrangements look polished and professional.