The main thing, when PRing, is to stay relaxed. BUT, how to stay relaxed may somewhat vary for different people.
What is same for all of us is that our spine should be vertical on chair. The reason is simply because brain gets most oxygen when spine is in natural position (not bent).
That's why Paul recommends that position in his book. The mind stays fresh and works the best when you're as straight as an arrow from butt to neck. 
And propping a book means not bending your neck (that much at least).
