Viewing Angle - Tilt or Flat Mount?

Anyone have a rule of thumb for determining need for tilt vs. flush mount? Mounting 50 inch plasma above a mantle - bottom will be about 5 feet off ground. Optimal viewing spot on a sofa is about 11 feet away. Other regular seating spots range from 8 to 14 feet. There will be no need to angle to one side or the other. Thanks
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Good thoughts on glare. I think I am ok, but will give this one a try. My concern has been more of comfort - neck strain, what have you. I don't think it will be an issue, but wanted to benefit from the knowledge and experience fo others.
FWIW - Ergonomically, a screen (any screen) above a mantel can cause neck strain since it is necessary to keep the head tilted upward (from your normal eye level gaze - even slightly)constantly working the muscles in the back of the neck to do so; especially if you work out the gemotery of the screen being 5' off the ground at only 11 feet away and youreye level being somewhere in the neighborhood of 45" off the ground. This is not a natural viewing angle for someone in a seated position. It may not seem like much of an issue but viewing a screen at eye level or slightly lower significantly reduces neck strain and minimizes the potential for chronic neck-back-shoulder problems.
If you are going that high up you probably would tilt it, but I would lower it if possible (in a perfect world).
Tilt & I'm saying that hoping you know there is flat (non-adjustable), tilt and cantilever mounts.
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