The chiropractors and dance medicine specialists that Diana consulted say no. Cracking isn't necessarily unhealthy, all you're doing is taking your joint to the end of its ROM (range of motion), which feels good and loosens it up a bit. The crack that you hear is caused by one of three (normal) possibilities: 1) a little bubble of gas escapes when you move the joint to the end of its ROM, 2) the cartilage is not smooth and makes a sound when it rubs against something, or 3) tendons or small, tight muscles make a noise when sliding over a bursa.
All in all, it's not necessarily bad to crack and pop joints if it makes you feel better and more at ease with your ROM. Cracking even releases endorphins, and while short lived, endorphins make you happy. So crack, snapple, and pop away! (oops, did I mess that up?)