mine is set up so that when it rotates, it switches off. when it has stopped rotating, it switches back on and the in-built degauss does its thing. that normally keeps it clean.
if we have a bit of a session with lots of different games being played some discolouration creeps in. if it was a big problem i could always install a hidden momentary off switch to straighten it up again. but i dunno, it kinda looks more realisic with a spot of purple in the corner, just like the busted up real ones i used to play (",)