when I have eggs that are really dirty that I want to candle I just take a damp washcloth or paper towel and carefully try and get the worst of the poo off the egg. I've never had any problems with that making them not hatch, and then I don't have to worry about the germs affecting the other eggs.
Yeah, I don't know what is best. I've heard "Wash them." I've heard "Don't wash them, you blooming idiot. That opens all the pores and lets in all the bacteria." So I don't wash them. I've also heard, "Sandpaper off the small poo marks." And I've also heard, "Sandpapering the poo marks just forces the poo deeper into the eggshell and closer to the chick which will infect the chick." And then everyone gives all their statistics and hatch rates, etc., that proves that their method of hatching is successful.

I think different methods work for different people because of a whole lot of varied environmental issues and situations. I haven't a clue what I'm doing. For me, incubation feels like I am driving blindfolded, with my foot tied to the gas pedal, my fingers crossed, and the radio on loud.