I personally avoid them, but some people scrape off what they can and incubate them with no problem.
Washing the eggs washes off the protective coating, and makes it easier for bacteria and disease to
enter the egg, however everybody has their own opinions about washing - I don't do it.
I spray them with Oxine AH and dab the bad spots. My birds eggs are rarely dirty enough to clean, but other eggs I have gotten did need cleaning. Oxine will kill most of the germs that can cause a bad hatch.
I wash all my hatching eggs, visibly dirty or not, in a hot dilute bleach solution before rinsing them off under hot running water. I get a better hatch rate when I wash them all. It also seems to help keep them viable for longer when storing eggs for any length of time. I've washed eggs then stored them for upwards of 3 weeks and still got a pretty good hatch rate from them. I know some people cite the 'protective bloom' as a reason not to wash eggs, but every single commercial hatchery washes their eggs and they wouldn't be doing it if it didn't work...
I purchased some bantam polish eggs off of Ebay and they came with washing instructions and were really dirty. I have never washed eggs before but I had never received eggs quite this dirty before so I followed the instructions but instead of using bleach water for the final rinse I used Oxine and they are incubating pretty good so far
but it is only day 5 for them so it is still to early to tell.