I always wash off saddle patch, girth area, bum and ears when I've ridden in the summer. I found that the collagen lumps (or whatever they are) on his back actually went completely when I washed his back every time I'd ridden with malaseb and warm water.
In the winter i only ever wash his saddle patch if we've been hunting but I do the same with warm water and malaseb again. In the winter I only use cold water for legs and tummy if its completely covered in mud!! I wouldn't want a full body cold shower in the depths of winter - I imagine my horse doesn't either.