I never wash eggs before I incubate them and I always have 100% fertility and started development. Only a few stop or die for some reason. I used to only put "clean" eggs in bator and would actually boil the dirty eggs and mash them up and feed them back to the chickens. But it got to where I was getting more dirty eggs than clean (my chickens freerange and get in nest with dirty/muddy feet, and some of the younger ones roost on nest boxes at night and poop in them) so I just started putting the dirty ones in the bator. Still having great results.
The only time I ended up with a whole batch of dead eggs was the time I washed them after the fact. When I gathered my first duck eggs (and the ducks lay in the mud and the eggs are filthy) I put the dirty eggs in bator to see if they were fertile. Once I candled and saw embryo development and movement I decided to wash the eggs. So I used a damp cloth to clean off the duck eggs and put them back and every embryo died. My chicken eggs in bator at same time which were not washed did not die.
I've had a broody hen lay her eggs on the coop floor, sit on them for 21 days, roll them around and everyone hatched. My ducks dig holes in the mud and lay their eggs and everyone I've put in bator hatches.
I believe that natural protective barrier is there for a reason and would not wash it off.