My roosts are 7 feet up! And my girls do just fine with them. They have a ladder to reach the roosts but when they come down they usually just flutter straight down to the floor. Below on L you can see the perches. On right, you can see the ladder.
They don't "need" to be, but chickens prefer to roost as high as possible. They feel safer high up. My coop used to be a hunting blind and it basically looks like a giant outhouse, LOL. It's 10 feet tall, 10 feet long and 4 feet wide. The roosts are actually structural--they hold the building together. So when I adapted it for a chicken coop, I put the ladder in, figuring that if they wanted to use the top roosts they could and if they didn't, they could use the ladder. They pretty much always go all the way up to the top roosts. See my Coop page (link in sig) for more photos, etc.
The only concern with roosts over 5 feet (my opinion/experience) is a risk of injury coming down. Mine are at 6 foot and I have had hens injure themselves coming off when they "fly down" instead of using the ramp/ladder. Thats my two cents worth.
Rule of thumb, high enough for at least one wingflap to soften the landing. Silkies and a few others excluded..........Pop
