There are no "rear" seakers in a 5.1 setup, you actually have "side" surrounds. If you go 7.1 that is when you would introduce a rear pair. Something like the Q series works great for a 5.1 setup, much better than a bookshelf in my opinion. Since you have 2 tweeters firing to the left and right, it totally wraps you in the experience, which is the point of a surround.
How are you planning on mounting them? If you wall mount them it is best to have them 2-3ft or higher above your ears when seated. Is the sofa going to be pushed right up to the rear wall? What is nice is that the rear tweeter will direct sound towards the back wall, giving you almost a 7.1 experience.
