Is cracking your back actually good for your spine? Is Neck Cracking bad?

I read in wikipedia that cracking your back actually allines your spine and is good for it, while cracking your neck is bad for your spine or something. Are these things true? Also, how do you crack your upper back? I have never tried, but it think this will make my shoulkder pain less acute. (Sitting at the comp 2 long) I can crack my lower back by holding onto the back of my chair and just turning around.