How straight should the back be?

It is common knowledge that the back should be straight, but no further details are usually provided.

In yoga, when doing seated meditation at the beginning and end of each class, my teacher usually says the following: on inhale, contract your groin muscles and try to expand the space between the top of your head and your hips, as well as between the top of the head and the shoulders. Then on exhale, release tension and let your knees drop.

It works great in yoga, which is more of a physical and body practice to me. However when I am doing half an hour of pure meditation, this whole up-down cycle is a bit distracting. On the other side, if I don't do it, it's all good and nice for a couple of minutes, but eventually my back starts to slowly crouch. Before long, I am totally hunched and need to correct the whole posture, which is equally distracting.

How do you deal with your back?