Great post! I've literally gone all out OCD by reading this thread, I was so happy when I got my current macbook I thought finally I landed a perfect one, I came home checked it out again and it looked great nice and white no yellowing that I could notice, then I thought to post a pic here again to see wabt ppl thought and when some said that it didn't look great I immediately started staring at my screen and all of a sudden I started seeing things but I now see that I'm seeing things that are not there, there is however a very very subtle light yellow tinge at the bottom but you have to look to really see it. I think this thread is making us all crazy and returning MacBooks which probably have nothing wrong with them.
I think if you use a black background and there is no yellowing/bleed it is a better test.
The white page will quickly "oversaturate" your retina and you will find it very hard to tell if it is really yellowed with a good screen. I am an ophthalmologist and a white background at full brightness is just going to overwhelm the rods and cones and cause some after-imaging making it very hard to tell what is white and what is yellow.
BTW, each eye will have a slightly different sensitivity.
IMHO, a truly bad screen will be more obviously esp with the black page.
As I look at some of the pics, it is my opinion there is nothing wrong with them.
Some are clearly very defective.
For me, I find the screens at 75%(except during the day with a brighter office lights) to be too bright. I prefer 50-60% is much better.
FWIW, no matter what brightness you are not hurting your eyes except to cause irritation with the overly bright background.