The Pen Displays are designed with a type of glass that is resistant to scratching and damage. No screen protector is required or suggested to be used with the Pen Display 24, and a screen protector might also introduce a high amount of glare to the surface of the Pen Display.
Adding anything between the Pen Display surface and the pen can cause the pen to work at less than optimum performance and introduce unwanted occurrences while drawing.
On a weekly basis, you will want to check your pen's nib to ensure that the nibs on your pen are not sharp or have worked into a point. The link below goes into more detail on what to look for when your nib starts to wear out.