I do sit like this on an aeron chair, leaned back as far as it can go. I almost never have back issues, but in this position (exactly like the far right picture in the article) my neck eventually starts to ache.

Good for back, bad for neck...

That's what I do, except I put my feet on a chair and keep my legs straight.

Trying to sit up straight makes my shoulders hunch up. Slouching lets my shoulders and arms relax. Sitting on a hard unpadded chair is good too.

Pretty interesting article.