For me, he'll do it WHILE doing whatever it is that I'm asking that he doesn't like or doesn't really understand/know well, and it doesn't really bother me all that much since he's still doing what I want with his feet. Pain is unlikely since he's done it bareback, in different saddles, etc, and it only comes up when he's showing some attitude. If I smack him on the side of the neck or otherwise correct him, he'll stop for a while, so it's not been something I've really worked to correct on a regular basis.
BUT I know some other people see it as a serious problem, and I suppose if I was interested in showing, it would be. Any safe ways to correct it? Any ways to correct it so he's less likely to try it with other riders?