Truthfulness is a very important Parami, because a big part of what we are trying to do is that we are trying to understand the Truth. In order to understand the Truth, we have to begin with Truthfulness.
Now, this can be applied on various levels. Truthfulness in our speech is often related to developing the Morality Parami, sometimes it is developing the Renunciation Parami: renunciation of falsity, renunciation of the ego. Sometimes we tell false things to others through a sense of sense desire, a wish for the benefit for ourselves; not seeing that this is not very useful to us. Truthfulness can go much deeper than that, it can go to Truthfulness to ourselves and to our inner direction - then it helps the development of the Determination Parami. Trying to be truthful to our direction, to be truthfulness to ourselves and what we are feeling, and what we are thinking. This helps to develop compassion to ourselves and wisdom.
Truthfulness is very much linked to wisdom, because a lot of the times when we are not being truthful to ourselves we are seeking the cause of suffering externally, rather than understanding the first factor of the Noble Eightfold Path - that is, Cause and Effect and, especially, the first two Noble Truths - that Dukkha exists and the cause of Dukkha is my own ignorance. So truthfulness helps with the development of wisdom. Naturally, if it helps to develop wisdom and compassion, it helps to develop patience; and also equanimity because if we are committed to the Truth, then we don't cover up things just to avoid the blame of others. We are truthful to ourselves and them, but it also has to be linked to compassion so that we can understand when it is appropriate to voice our truth. We don't have to share our thoughts with everybody, especially if we are having negative thoughts about someone and then they ask us, "What do you think of me?", then it is better to link it with Compassion and Lovingkindness.
I'll give you an example of this. One of our assistant's girlfriend came to do a retreat, and we actually saw that this person was not very compatible with our assistant, especially on the morality level, and this person did not have the wish to change their views either. And at the end of the retreat the assistant came very excitedly to Steve and asked, "So what do you think of my girlfriend?" Steve said, "I think that it is more important, what you think about her, don't you?" So it was linking Truthfulness to Compassion/Lovingkindness. Apparently later, the assistant saw for themselves that they were not compatible and the relationship did not continue, so we did not have to say it to them at that time.