Misting eggs is to slow down the moisture loss during incubation. It is not always necessary- but gauging form the size of the air cell ( or even weighing the egg to see what % of weight they have lost. )you can tell if the eggs have lost too much or not enough moisture. Duck eggs are more porous than chicken eggs- so some say there is no point to misting - but I do give eggs a mist when the air cell is looking larger than expected.
You should start misting on day 10 from what I read as misting changes the egg white consistency so that when they hatch they don't end up being covered in thick gooey stuff. I have tried both and I would definitely mist once a day with luke warm water when you turn your eggs if you are hand turning.