The weather is finally going to co-operate for everyone to get their first bath by the end of this week
I don't show, so I might
use a shampoo on them once a month if they haven't been trailered somewhere to trail ride.
If I go trail riding and they've picked up a lot of ticks, I will bath them with some sort of iodine shampoo; it makes the ticks stand up, therefore much easier to see and pull off.
Like others have said, I will hose them down and squee-gee them off as-needed, and that is generally every night during July & August. Middle Tennessee gets sticky/humid during those months