Jumping back into the discussion.
As for horses it really depends on the horse if and how often you have to clean their sheath. Some of our geldings we had to clean every 3 or 4 months because the build up was awful and some we never cleaned because they never got bad. Some of our boys love having it done and some absolutely hate it, and I'm not about to put my hand there on a horse that isn't excited about having it done, no pun intended. That being said it still is a totally gross task that i try to do as infrequently as possible.
One clue that they needed to be cleaned was the sheath would actually start making a weird sound while they are being ridden. I know that one of the reasons its important to clean a domesticated horse's sheath is the amount of arena dust that they are exposed to ( if there are ridden in a sand ring) does make the shmegma build up way worse. Our non arena ridden geldings rarely have to be cleaned if ever. One weird thing I've noticed over time is that light color horses usually seem to be worse in the schmegma department than bays and chestnuts.
I have seen infections in older geldings from not having their sheaths cleaned and have problems "dropping" to urinate. Also if large pieces of foreign matter does work its way up in there it can cause an infection to and cleaning the sheath helps to get any stray shavings, hay, etc. that might have found its way up in there.