You'd be surprised how much I don't know about A/C /forums/images/graemlins/grin.gif
You may be right on Texas weather, I don't know.
Couple of years back working on the inside, did about 400 load calculations a year (Wrightsoft) and only had four that came back due to issues (due to change in work order on the structure, contractors can be a pain to deal with).
What I will say though is that if you have a good HVAC contractor, and he does his job right, if he gives you a load for the structure and it doesn't heat-A/C properly, he would change the system out at no charge (there is "fine print" there) homeowners can be interesting dealing with temperature (keep in mind, changing the load may also require changing the ductwork, which alot of times never happens).
I deal with a couple of contractors who do this on a regular basis, may charge a little more than just a "change out man", but so far these people have never had to replace a system due to the load requirements of the structure.
Considering the fact that Centex homes, Lennox, Goodman and American Standard have home offices in Texas, I would be curious how load calcs are done down there.