Before every side and after.
+1 for melamine foam (Magic Eraser material) available as thin sheets that lay perfectly on the platter. I learned this technique from Joseph Long (groovetickler) who has seen and repaired lots of cartridges damaged by liquid and gel cleaners. Microscopically, melamine foam has lots of open spaces to grab particles, much finer than even the finest brushes. Dropping the stylus into it causes dirt, etc., to be trapped in the foam and it leaves no residue to fuse to the stylus during play.
100% agree with comments about insuring the cantilever is free of debris before wet cleaning. I wet-clean very rarely, then finish with dry melamine foam.