You must release the top nuts that hold the top of the shock to the body while the vehicle is on its tires. Then, when you lift it, the trailing arm (rear) or lower control arm (front) will continue further on down and stop on the subframe (rear) or on the body (front). The shock is now free to be removed. The spring has no force on the shock. Normally, when you lift the vehicle, the wheel continues downward until the shock stops it's travel. It would be dangerous to attempt to remove the shock in this condition.

In the front, you can change quite a few things without a spring compressor. Lower control arm bushings are one of the few items that require it.

In the back, it's the same deal. Nothing requires you to remove the spring, unless you are going to do trailing arm bushings. I will say, however, that it is far more pleasurable to do subframe bushings with the springs removed.