I remember cleaning the ball once or twice, yes, when it got too bad to ignore.

Mostly the gunk accumulated on the rollers, though (and under the mouse, like in optical ones), and that was easier and faster to just scratch off with your nail... except for the little third wheel (seen on the bottom left picture) that kept the ball centered, of course, because that one wasn't only smaller but on a spring, making it almost impossible to scratch the gunk off of.

There was also the occasional hair tangled up on a roller, of course, which was almost impossible to remove. Those you just pushed aside onto the roller's axle and hoped the mouse would die of some electrical failure before the poor thing got too full of hair to roll.