I brush Hoove's, with a pin brush. I start at the bottom with a small piece, and then slowly add more hair to it as it smooths out. I don't tend to get a lot of hair out of him, but I keep it pretty tangle-free so the brush just slicks through. His tail is almost to the ground, and I really don't seem to be impacting it's density much. I do stand behind him, but I know him pretty well. We have long sessions out in the pasture of me leaning against his backend with my head on his butt and take naps.
But as it was said, it's a person preference. It seems to also depend on the horse's hair quality...Hoove has really thick hair, so he doesn't seem to lose a lot of hair from it...but if it seemed he did, I probably wouldn't brush him as often.