I just washed my horse's tail. It was really warm for this time of year here (11 Celcius = about 50 F). I filled a bucket up with warmish water, added a bit of shampoo and just swished his tail around in there. It wasn't perfectly clean, but it was better than it's been for a long time (he's a grey)! I didn't wash the tail bone and won't do a full body wash until spring, but I did spot-wash his body. He was perfectly fine. I don't see why anyone would not want you to wash hairs he won't even feel. Now if you're talking about the tail bone as well, maybe that's different.