We normally get a feel about the pups when they are born as well. We watch them off and on between then and 8 weeks to get a feel for their personalities etc. Even though we might evaluate at 6 weeks... At 8 weeks is when we will really evaluate them and possibly return at 10 weeks before they head home. We find between 6-8 weeks is when they have their "awkward" stage, so thats why we wait. Even though it takes a lot of patience :)
Five weeks and I have picked the best pup out of every litter I have looked at at five weeks. In my experience they can change tons after that but they will come back to what they were at five weeks. Most people pick at 8 weeks.
I think it depends on the line. I will look at them each week but usually my pick stays consistant from the 6 wk eval. I do think it is easier to spot short upper arms closer to the 8 wk point. They are always there of course...it just becomes more noticeable as the puppy begins to grow.