Also, puppies develop at slightly different rates, so while 7 wks might be the best age to test most puppies, there will be those that probably would test better a few weeks later. Certain traits may be better tested at different ages than others, or perhaps tests should be repeated at specific ages to improve predictability. Finally, how the breeder raises and handles puppies prior to testing has an impact. My puppies are so used to being upside down or dangled by the time they are 7 weeks old, they accept if from just about anyone. They hear lots of loud noises in the whelping box, so it is sometimes hard to test noise sensitivites without knowing what a unique sound might be for them.
Puppy tests can be a useful guide to evaluate puppy temperaments at a certain point in time.
I agree, though, the observations of a good breeder over a period of several weeks is far more valuable in selecting a puppy for a particular task. I may not agree with the tester's evaluation or might see something I find significant that the tester completely missed. But I still would rely very heavily on the breeder's observations, above and beyond the test. I used to have a friend test all my litters for me. It was extremely rare that she found anything out about the puppies that I hadn't already picked up on through daily observations.