If they are going under a broody, I wouldn't clean them. But if they are going into an incubator and the incubator is sterile, go for it. I always clean my incubator eggs. I gently wipe them with water just a bit warmer then they are. Don't use soap, and don't sumerge for any longer than 2 minutes. Since it does remove the outer membrane, bacteria can get in a bit more easily, but as long as your incubator is clean, you won't have a problem. Like I said, I always clean my eggs and usually get a 95% hatch rate.