My barn doesn't have an indoor wash rack, so I keep her tail up in the winter. The reason - one of the first years I owned her, I went out to the barn right around Christmas after not being out for a week. Her tail had gotten caught up in her blanket leg straps, and was acting as a shelf for the poo to sit on. Frozen poo tail. So gross. And so hard to clean in minus 20 weather. So ever since then I put her tail up when I put her blanket on in the winter.
But in the summer I don't worry so much about her tail as I can wash it frequently. She still pees and poops all over it when she wears her fly sheet, but I can wash it twice a week if need be.
So my favourite shampoo? Mane & Tail Spray 'n White Shampoo by Straight Arrow.
I just have to watch that I don't drop the bottle on the ground from the top of my tack box and smash the sprayer. Cause that will happen if you do that.
I currently have a three year supply in my garage - bought the last couple of containers from a local store that was liquidating them at half price. Should be just in time for the boy horse to get a white tail.