there will be no set #. 1 factor will be the viewing angle of your tv. If you have a laser pointer, sit/lay how you be viewing the tv, have your wife put a pincel dot on the wall right where you would like the center of the screen be, then switch spots.
If there is a big diffrence in viewing angle, your going to have to see if the tv's viewing angle is with in that range.
If it is not a big distance in what you & her want then spilt the diffrence or you can say the stud in the wall moved over a little & it MUST be but 'there' or the tv will fall or something like
I will be hanging my 32" LCD on the wall this weekend. What is the preferred hieght for wall hanging. Most viewing will be lying on the bed which is directly in front of the TV. Right now, I have the TV on the stand on a piece of furniture that is 40" high. Seems to be a good height for viewing from the bed. Wife wants it much higher. The 40" piece of furniture will be replaced by a much lower cabinet that house the Cable box and DVD player. I will be putting an electrical outlet behind the TV and will run 2 HDMI cables inside the wall. Should be fun.