It is important to learn yoga techniques from guru face. Yoga asanas, prayanayama and meditation when practiced in the presence of a guru can help you understand the exact nature of your doing asanas, meditation and pranayama.
When practicing meditation, especially kundalini meditation, there is emanation of strong psychic energy, which you may not be able to appropriately control. A knowledgeable guru can get a sense of any abnormality in your meditation and can intervene if necessary.
Though you can correct your postures while practicing before a mirror, it is important that you have the support and guidance of an experienced yoga master to guide you through the process and rectify any anomalies in your posture.