My three year old gelding is so dirty all the time. I didn't really start paying attention until this year. No one I know cleans the sheaths themselves. They get the vet out to do it. Except my trainer, but he used to show/breed so it's just standard procedure for him.
My poor guy. He was always rubbing his butt on things. I thought he had pin worms, but when I finally started learning about sheath cleaning, and looked, he was filthy.
I cleaned him for the first two months ago, and he is already super filthy again. Worse than before, actually. I have a feeling he's going to need cleaning once a month or so to be comfortable.
I check my mare's udders every once in a while when I remember. Usually it is just a little crusty build up. Nothing bad. She lets me pick it off without a fuss.