I have a question about stability. I'm going to build a simple free standing book case 6' x 3'. It's going to hold sheet music on one or two of the shelves so needs to be stable. I want to have a pole that keeps things from sliding off behind the book case. My question is if picture #1 where the pole going across the width in the back just above the shelf is as stable as picture #2 where all 4 poles are at shelf height. The 3rd image is the shelf idea except I'd have 5 shelves. Thanks!