I think it can depend a bit on the horse as to what is best. 7 or 8 years ago my mare had mud rash going into winter and the vet suggested just hosing the mud off her legs when she came in at night and towel drying. I'd always been taught not to wash legs but actually, her legs healed up on this routine and she didn't get any mud rash then or the following winters on that yard.
I've sometimes wondered whether washing legs is more of an issue for horses with hairy legs. If a horse has a lot of feather, they could be covered in mud but the skin underneath the feather will be clean/dry. Whereas if you hose their legs, then the skin will get damp and take a long time to dry, making mud rash more likely. Whereas horses with no feather and fine hair will have wet, muddy skin when they come in as they have no protection, so actually hosing their legs may be the best things for them. Of course, it's not fool proof - some horses are more prone to mud rash than others, definitely.