I was reading "Tricks of the Trade" by Pat Hastings and she repeatedly says that a puppy's structure at 8 weeks old (give or take 3 days) is what the puppy will be as an adult. Often, I hear breeders say that they can't tell what the puppy will be as an adult, until the puppy is several months old (atleast).
I have found that with GSD's it can depend on the bloodline. With my dogs being linebred on Sundance Kid they can go thru some real uglies later in puppyhood, so I like to choose early, around the 7-8 week age. Other bloodlines seem to take longer to develop, and their breeders like to choose later. I have had pretty good luck with it.
Personally I like to see puppies right at birth while they are still wet and not full of milk.....you can see so much of their structure and I tend to see it as pretty accurate. Of course within a half hour they are fat full of milk and the window is gone lol But, I do like to look at them at birth.
I generally stack my working line puppies at seven weeks, taking photos of stacks and heads (to get an idea of earset), any later than that - its too late IMO. One thing I have noticed, the better the movement (and choice of gait) in a natural setting (i.e. just observing in the puppy play area as opposed to on a leash), the more correct the structure - from the get go.