Is it wise to be truthful? Yes, if wisdom is about having the mental skills to enable living a long, full and best kind of life, extending that to include others where possible. Regard for others is going to be part of that - doing to others as we would that they do to us - so since we need people to be truthful in their communicating with us then we too would be wise to be truthful in communicating with others. Another factor is the power others, if they are in authority over us, have to curtail our happiness and even shorten our lives, so wisdom will avoid displeasing those with such authority and power over us. Of course that is most pertinent to the most authoritative of all who many call God who can curtail this life and the next and curtail our happiness and fullness of life even while letting us live now or in any afterlife. So pleasing God is key in wisdom for those believing in God or gods. This too entails truthfulness as do all matters pertaining to those with power over us who usually look poorly on our dishonesty. When we are made wise enough by God to realise God is truthful and hates lying we get to desire strongly to be truthful somehow. In John's gospel (chapter 8) we find a hugely wise discourse in which Jesus Christ describes God's primary focus on truthfulness and the wise counsel that lying and truthfulness are like opposing languages, lying being native to Satan and children of Satan but God only owning as children those who speak the language of truth and desist permanently from lying.
