IMHO:
Sit in the desired viewing position and place the plasma at a height where you are neither looking up or down at it. In other words at a height where your head is neither pointing down or up. This is what the ergo specialist at work told me to setup my computer display since it will cause the least amount of stress on your body.
Having said that, it's only my opinion. And you know what they say about opinions...they are like butholes, everyone's got one.