It is definitely worthwhile for people to do some research on the correct structure and conformation of a GSD. I am a working dog person, but I have handled many dogs in the show ring. I have shown my own dogs to get a breed survey and Koer rating. I have handled dogs for others at regional and the National Sieger Shows.
Extremes in any aspect are not practical or desirable, IMHO. Extreme angulation which may be nice for a trot is not going to be great for jumping and holding up over years of working. A dog lacking any angulation is not going to hold up either. There are many pictures posted as examples of dogs with straight backs, which are supposed to be good examples of structure and conformation? Honestly, some aren't very good examples of what a GSD should be, should look like, structure, conformation or a dog capable of doing any work.
It takes a little skill to stack a dog properly. A working dog should have some angulation, not extreme, but some. The stack can be exaggerated by drawing the rear leg back too far, or not far enough, by not having the front legs straight under the shoulder, by not having all four feet positioned properly, etc.
Looking at a dog in a "natural stack" or just standing still often tells us very little about it's conformation. Because a dog has little angulation does not automatically make it a "superior" or more desirable GSD than one with more correct structure, and vice versa. It is so much more involved than that.
I look at the overall dog. The truly great dogs, IMHO can easily do the AD (12 mile jog next to a bike), can jump the 1 meter hurdles with ease, clear the a frame, do very nice OB, and can do other work, in addition to having correct conformation, temperament and drives. It is a total package, not just how the dog looks or how straight it's back is. A straight back is not necessarily a desirable structure and be very leary of breeders that are marketing dogs with "straight backs." It goes along with oversized, "old style", blah blah blah. A sign of a breeder that uneducated, inexperienced and not very knowledgeable.