Of course one cheat would be to detect the when you switch to a "hard" coin that has lower shares per time and go elsewhere until it switches to an "easy" coin. Then come back full force. Then get paid off per share at the same rate as the guys that stayed the whole time.
So "hard" coins are ones with high reject rates?
I don't consider that cheating. People should feel free to do that. It won't lower anyone's profits. In fact, it might increase them.
But there's really no reason to, unless that particular miner has much higher reject rates than others in the pool.