I like to take advantage of all pieces of greenery if I can, even the shortened pieces of lily grass. Cut the bottom off at a sharp angle for easier insertion into the wet foam.
Follow the angle of the other greenery - straight up in the center, then begin angling outward as you move towards the edge of the arrangement.
Finish off the top with another sharp cut if needed to give a clean look.
See how the shortened grass stems balances the loops inserted on the other side.
A side view gives even a clearer look of the insertion of the grasses.