It depends on your chickens a little and on your coop a lot. In general, as low as reasonable is best.
I think it is grossly blown out of proportion, but many people are concerned that a heavy breed chicken can injure its leg hopping down from a roost. My chickens spread their wings and fly down to soften their landing, but you are dealing with living animals so anything is possible. But the higher the roosts the more horizontal room they need to fly down without banging into things, so if your coop is small, lower is better. They are not necessarily the most graceful of fliers. With that said, my full sized dual purpose flock will sometimes launch themselves from a five foot high roost, fly forward about 7 or 8 feet, hang a sharp left to fly out the people door, and land in the run. Anyway, I suggest as low as reasonable is best.
The main thing is that the roosts need to be higher than anything you don't want then roosting on or in, especially nest boxes. Most chickens instinctively want to sleep on the highest thing available. They poop a lot when on the roost and you don't want them to sleep and poop in the nests.
My normal progression in determining roost height is to determine floor level, determine bedding level, then set the nest boxes. Once that is done, put the roosts a little highter.
Another consideration with me is that I really like permanent ventilation open in the coop, especially in winter. I put mine up under the overhang. But I don't want them sleeping in a draft in winter, so I keep the roosts low enough that they are sleeping below those ventilation openings.
Good luck with it.