Back when I was a teen in Pony Club, it was necessary to clean my gelding before each event because it was checked. He was never very bad but I think I own 2 of the grubbiest guys ever! Even my vet said he's never seen a horse who produces so much gunk. My pony and my draft cross get gunk on the insides of their hind legs from when they hang out and I clean them at least 2 times a year...... our paint isn't too bad which is good because he can get testy about me being so familiar..... I usually put on a glove and grease them up with KY jelly or baby oil a day or two before I plan on getting to it and that does help soften things up, then use Excalibur or Ivory/Dove some other mild soap. My POA developed a squamous cell carcinoma on his "bits" about a year ago and had to have it surgically removed. In researching it, I found studies that said a dirty sheath was a cause and then other studies that said that wasn't a cause...... so I don't know but I figure I want to keep checking him for a recurrence so it's a good idea to clean him regularly.