My boy is white all along his back and in the summer he likes to roll around a lot in the dirt. Then, when I ride, the sweat really brings that dirt up to the surface and gives him a grimy spot at the rear of my seat area, under the saddle pad. I'll often blast/hose this off after riding (sometimes daily). Although, I don't think it would be good to bathe a horse daily with soap because it removes the natural shiny oils from the coat (like Blu said). Could dry out his skin.