I'm starting to think that idea is just a matter of opinion. From what I've always heard, wetting down the area around the water tub for the horses feet is just an old wives tale. Do a search on it and you'll see people that think both ways so who knows. Whatever works for ya, I guess. I don't like to do that but I forget to turn the water off so much that it's kind of happening anyway. George is so prone to thrush that I really do prefer to keep his feet as dry as possible.
I don't think leaving a little mud around the water trough makes a bit of difference on feet... but I wet down the paddock when it's dry just to keep the dust down.