You have all the time to rearrange or polish your flower arrangement. The flower design should look beautiful and artistic enough before you place your filler, when the filler is already placed, it would be harder to move your things around the area or place. Bear in mind that The filler are very fragile.
To get a beautiful flower design or an arrangement, you don't need to show all of these three. The most important here is that your design has ornamental flowers; you will still get a terrific and an artistic flower arrangement.