Cracking your back sends nitrogen and carbon dioxide bubbles from the tissue surrounding your spine into the facet joints that link your vertebrae. Those bubbles cause the joints to relax, which makes them more susceptible to injury if you happen to exercises soon after. Worst case, you could end up with a herniated disk.
To play it safe, work out the kinks by stretching instead. Lie on the floor with your hands behind your head. Keep you back straight and tighten your hip and abdominal muscles; hold for 5 to 10 seconds, and then relax. Now pull your right leg toward your chest while keeping your lower back flat and your head on the floor. Repeat with your left leg.