In theory, the most comfortable viewing height would be a height that minimizes the extent possible neck and eye strain. Therefore, you would want the center of the screen to be roughly at eye level. People who mount TV's above fireplaces always baffle me unless you have a recliner and your sightline is angled up...
So, in ideal conditions, for an 82" TV, the bottom of the screen would be about 21.25" from the floor assuming an average 42" eyelevel seated height... this doesn't give you a lot of room for a center speaker or entertainment unit.
WAF and aesthetics can also come into play. I like my TV as centered as much as possible between floor and ceiling... Assuming an 8' ceiling, that would mean mounting the TV at about 27.25" above the floor. The further you sit back, the less extreme the 6" has on your viewing angle so this would be quite acceptable.
http://www.sanus.com/en_US/heightfinder/results/samsung/82/UN82MU8000/vxf220-b1/centered/42/0/in
