Posture is not simply a matter of standing up straight, like your mother told you to; posture is created by the Hamstrings and Hip Flexor (mainly the Iliopsoas) muscles. If the Hamstrings are stronger than the Psoas, the pelvis tilts backwards and if the Psoas is stronger than the Hamstrings, the pelvis tilts forwards.
The spinal column balances on top of the pelvis and adjusts its curves according to how the pelvis is tilted. This is the body’s internal balancing mechanism and consciously tilting the pelvis backwards or forwards does not play a role in determining your own natural pelvic tilt whereas the ratio of Hip flexor to Hamstring exercise definitely does – Physiotherapists use this method when they correct faulty posture.
The implications for yoga practise are large: those who have weak hamstrings as a contributing factor of their lower back pain are helped by yoga teachers who like to give a lot of Virabhadrasana, standing splits, Ardha Chandrasana, etc. But for those who have stronger hamstrings, this will cause greater posterior pelvic tilt and pain, they need postures like Utthita Hasta Padangustasana and its variants.
Teachers cannot know where the balance is for individual student but they should aim for a balance when creating sequences: for example, asanas where the leg is lifted up to the back and when it is lifted to the front should be in an equal ratio.
A misconception also commonly exists about Virabhadrasana and its variants, namely that these strengthen Quadricep muscles. While there is a contraction of the Quads, the Hamstrings and Gluteus Maximus are contracting eccentrically to control the lowering of the Pelvis and as such, support the weight of the torso.
This is true of all lunge asanas. Knee extensions in asanas like Titthibhasana are the only true Quadricep strengtheners. Iliopsoas strengthening is needed to balance Hamstrings and most teachers do this with Urdhva Prasarita Padasana but again, in what ratio? Standing balancing asanas in an erect position don’t affect pelvic tilt and are very good for balancing the legs and back.
It is interesting to note that Iyengar, in Light on Yoga, doesn’t give much attention to standing asanas but considers them suitable for beginners. Ashtanga yoga sequences also do not place a large emphasis on standing asanas; the standing sequence is a small part of the overall practise in all of the series. One has to wonder whether the modern, western obsession with leg asanas is wise, given the fact that this can alter posture, not necessarily for the better.
Reading sources: Kendall, McCreary, Provance, 1993, Muscles, Testing and Function BKS Iyengar, 1966, Light on Yoga