The general rule of thumb is.. if your knees crack and it hurts, then it's a problem. My knees crack all day long because I've had surgery on both, but it usually doesn't hurt unless I've been really active on my feet. If it hurts though you might want to consider an exercise that doesn't rely on 100s of pounds of weight pressing down on your body. Some people will advocate that everyone can do squats but even with correct form, some people have knee or joint problems that get in the way. Try biking, swimming, any bodyweight exercises (such as gorilla hops) to improve leg strength, then ease into the weights.