Only IF you want your centers to appear tighter and filled in, here is my tip:
Take a strip of paper about 1/2-inch (13mm) long and roll it tightly. The bigger the opening, use a longer strip of paper. I use a quilter's or shirt pin to get it tight, but you can use your quilling tool or fingers. There is no need to use glue as the coil will expand and stay put once you place it in the opening. The easiest way for me is to insert it with my tweezers.
This is the result. You may like the look of having an open hole for your project. But if you ever need to fill in one or two in your project, try this technique.
Have fun quilling!