I also use a microfibre cloth slightly damp with glass cleaning solution, and afterwards wipe it dry.
An added note: It is possible to get crumbs or lint embedded into the space between the edge of the case and the touchscreen, making it go unresponsive (due to the added pressure of the "crumb" between the casing and the touchscreen), or it may get sandwiched between the touchscreen and the reading screen, requiring disasembly of the unit to get it removed.
I suggest that special attention should be paid to the edges, by cleaning the screen towards the center, and checking afterwards that nothing got sandwiched in the border
Edit: I have encountered both conditions... the link to the remedy is
https://www.mobileread.com/forums/showthread.php?t=76265