The bacteria that causes mud fever lives in the soil, so to my mind it has always made sense to wash it off. Legs are hosed off top to bottom every evening, and are always dry and warm by the time I do lates at 10pm ish. I've never had a problem with mud fever in horses I have managed this way.
Nothing, but my horse has loads of feathers and is always clean and dry underneath whatever the outside is like. A few times during the winter (maybe once every month or so) I like to wash the legs thoroughly, let them dry somewhere clean and then put on pig oil and sulphur. I haven't been able to do that this year as I don't have any stable at the moment (so can't dry them in a clean environment) and he has been fine, though we also have shallow peat here rather than the deep clingy clay mud of my previous home in Somerset.