I usually give the eggs ours lay a quick rinse and pick off bits of mud or poo.
We don't incubate them, just eat them ourselves or sell the surplus to friends and neighbours that call round. I think I read somewhere that you're not supposed to wash them if you take them anywhere to sell - same as you're not allowed to call them free range.
The question was about washing before incubating.
But in terms of washing for eating - the danger is that you can end up washing the dirt
in through the shell and make the previously hygienic yolk-and-white contaminated with all sorts! If you really want to wash (rather than just wipe with a dry cloth) then make sure your washing water is
warmer than the inside of the egg, which in theory reduces the amount of ingress of water into the inside of the egg.
The other thing I've heard about washed eggs is that the impervious film on the shell is removed by this, so subsequently germs can enter the egg through the shell.
For eating eggs, it's best not to wash, really