You want to avoid locking your knee joint in any standing one legged posture such as Tree Pose (Vrksasana) or Warrior III (Virabhadrasana III)
Locking your knee takes out the muscular effort from other parts of your body and puts all the pressure on your joint. Whereas micro bending your knee engages the muscles surrounding the knee joint.
With repetition and age locking your knee joint in standing balancing postures could be a problem as you grind down the cartilage and over stretch the ligaments and tendons on the back of your knee and inside your hips.
This comes from a longevity point of view so micro bending your knee in standing balancing posture is going to give you a healthier practice.