You want it so that either the top of the screen, or the centre of the screen, depending on your preference, is at eye level. I would be leaning towards having the top of the display at eye level. Mine currently has the centre at eye level and I wish it was a bit lower.
As for viewing angles, it depends on the panel type that an LCD is using. It ranges from being perfectly acceptable for most people (tends to be IPS) to having huge contrast, colour or gamma shifts that look awful at an angle. You need to try the set you're thinking of buying to see if it's acceptable for you. Plasmas are mostly unaffected by viewing angle.