You absolutely do not have to always route around inside their sheath. I wouldn't even present that as an option with a mini horse. It is plenty easy to wash without actually "invading" their sheath. I find the hose a far less prefered way! lol Imaging getting a cold water enema... that's basically what you are doing! Even worse, because the penis is a very sensitive organ.
I'm not talking about breeding horses most of the time. Breeding stallions (regardless of closed herd or not, infection is infection and its not caused by "bad" stuff, just the normal junk present in their body and foreign to the mare's uterous) are different and should be kept even cleaner than a gelding.
I too hope that horses that aren't cleaned don't make much smegma, because it needs to be removed. Natural horses take care of it themselves. When we take that option away (through castration or whathaveyou) we need to take over and maintain their health for them. Its just another routine part of male horse ownership.
And also don't forget that smegma and peeling skin (the outward signs of a dirty sheath) are only half the problem: the biggest problem is the one people don't see, that pesky bean. Smegma makes things uncomfortable and filthy and terrible smelling if ignored, possibly leading to skin irritation and drying if left alone for WAY too long; the bean is what causes the pain and difficulty urinating, and that is hard to notice unless you look hard, or spend far too much time between the legs of male horses like I do, lol.
My point is besides breeding stallions, to all those other male horses that don't have the luxery of cleaning themselves inside a mare. (ick, now that's a disgusting image! lol)
I guess I live in an ideal world
I get to see the cultures come up on horses that haven't been cleaned, which sometimes have huge infections that you wouldn't even notice by looking. And I've seen the opposite too, from horses that were cleaned too much with harsh chemicals that had the infection (FROM the cleaning) go up into their tract and give them a perminent reproductive tract infection. Not pretty; something that can be easily avoided through careful and responsible management.