I think it's more up to you what you choose to do. If the egg is visibly dirty, I would wash it. Warm water on a paper towel would be fine. If it's really poopy though, I'm not sure I would try to hatch it.
A dirty egg has the capability of contaminating your incubator with bacteria as well as the eggs themselves internally. But quite a few people say that washing them (with soap/water etc) will remove the out protective coating from the egg and cause bacteria to be able to get into the egg too. I think this should not matter if you have a clean incubator.
Eggs you purchase from the store are washed (yes the ones you can purchase to hatch at the whole foods) Major hatcheries wash and sterilize their eggs before incubation as well because it offers them high hatch rates.
I really would like to test out this theory with a big lot of eggs and see it it really does affect the hatch rate. Wash half and not wash the others and put them in separate clean incubators.
Hope that helps?