I can see a hairline crack on the chewing surface of my tooth. Does it need treatment?

It depends on whether you have symptoms. Fine surface lines with no pain (enamel craze lines) are common in adult teeth and are often simply monitored at regular check-ups. However, if 1) you feel a sharp pain when biting down or releasing, 2) you get sudden sensitivity to cold, or 3) you find yourself avoiding chewing on that side, it may be a cracked tooth in progress, and covering it with a crown before it splits further is needed. If a crack reaches the root it can lead to root canal treatment or even extraction, so have an examination if you have had the symptom even once.