I clean my mare's udders about one a month. Gunk from sweat, dirt, or other mysterious thing get stuck up in there and it is good to get it all out every once and a while. If your mare threatens to kick you, attempt to clean more gently, with drugs, a combination of the two, or not at all. It is a very sensitive area for the mare (as one may expect) and she won't take it lightly if you slip.
My mare is very good about it and tolerates it nicely, but I have seen some mares flip over in the crossties in an attempt to protect their goods.
To clean, a generally only use luke warm water and my hands. One may be able to use soap, but I have never felt the need to.