The practice of the fourth precept aims at inculcating a respect for truth in the mind, implying both one's own obligations as well as the rights of other people to truth. This is one of the most important components in developing sound social relationships, and it makes all documents, contracts, agreements, deeds, and business dealings meaningful. When we resort to falsehood, we not only become dishonest but also show disrespect to the truth. People who tell lies discredit themselves and become untrustworthy.
It is true that sometimes telling lies may prove more profitable than truth, especially from the material point of view. Because such gains are unwholesome and may cause harm in the long run, and because material profits are likely to lead to more falsehood and fabrication, it is imperative that the practice of the fourth precept be duly emphasized. Where a person's reputation and feelings are concerned, discretion should be exercised. Of course, there are instances where silence is more appropriate than speech, and one may choose this as an alternative to prevarication and falsehood.
Motivation is an important element in determining if one is transgressing the fourth precept and whether a given verbal expression constitutes a kammically unwholesome act. For instance, when an event is fictionalized for literary purposes, this may not be regarded as falsehood as such for the intention of the work is obvious and there is no attempt at falsification involved. Another example is the case of an invective, where an abusive expression is used (such as angrily calling someone a dog). This is a case of vituperation rather than fabrication or falsification, although it is, nonetheless, a kammically unwholesome act. Also, there is a clear distinction between expressing untruth with a selfish intention and with a well-meaning motive, as when a concocted story is told for instructional purposes or a white lie is told in order to keep an innocent child out of danger.
These latter two instances are even accepted as illustrations of the employment of skillful means. A story is told of a mother who returns home to find her house on fire. Her little son is playing in the house, unaware that its burning roof could collapse at any moment. He is so engrossed that he pays no attention to his mother, who is now in great distress, being unable to get into the house herself. So she calls out to her child, "Come quickly, my little one, I have some wonderful toys for you. All the toys you ever wanted to have are here!" In this instance the mother is using a skillful means that eventually saves the boy's life. Under certain circumstances, this may be the only alternative, but indiscriminate use of such means may lead to undesirable results. One needs to be judicious, therefore, in the practice of the precepts.
Sometimes speaking the truth may cause more harm than good, especially if it is done with malicious intent. A vindictive neighbour who spreads the scandals about the family next door may be speaking the truth, but she is neither doing anyone a service, nor is she practicing the Dharma. A spy who sells his nation's sensitive classified information to an enemy may be speaking the truth, but he could cause much harm to his nation's security and jeopardize many innocent lives. The Buddha says, therefore, that one should speak the truth which is useful and conducive to the Dharma, and should avoid that which is useless and is likely to cause unwholesome karma to oneself and others. (source: web.singnet.com.sg)