How important is warming up prior to lifting for cartilage, ligaments and tendons?

I keep hearing a lot of information that conflicts about warm ups and stretching prior to lifting.

My question particularly concerns my knees during the squat. I've noticed some evening tenderness and clicking in my left knee. I usually keep cardio to the elliptical, and my lower body deadlifts, squats and hamstring kickbacks.

Should I be warming up prior?