I would definitely clean the sheath...the horse I show at the barn is a big gelding with a really *large* sheath (not a medical issue...he and his full brother have the same big sheath), and it seems to get dirtier than most horses because of that. I noticed his tail head was rubbed so much that it was nearly bald, so I knew he'd been scratching. Since then I've been cleaning his sheath once about every 2 weeks and it seems to have helped. His hair is growing back, anyway, so he must not be rubbing as much.