I've only just discovered this amazing blog from a Yogini in Virgina, USA, "Just here, Just now," so I thought I would share.
The latest post is all about sitting out of postures. Beautifully written and true to the point. Many people feel ashamed to sit out of a posture. It is okay to take a knee. Be honest with yourself, if you need a few moments to rest, get your breath back that is fine. It is recommended not to sit out of both set's of postures and at least have a go at one of them. Most of the time getting back in makes the crazy feelings pass.
I especially love the last paragraph in this piece. If you are going to sit out, do so nicely. Flapping & thrashing about, drinking water & generally exerting more energy than you would doing the posture is counter productive. (& cause you neighbors to start grumbling!) If you have enough left in you to do the above, why not give the posture a crack or sit quietly and allow your body to recover and gain some energy for the next set.