I believe keeping to the truth, even on the small things, is important. Even without taking into account the moral issues, I find the practical evaluation of being truthful to be straightforward:
- Reputation is important and valuable. If you say things that are untrue, even small things, and people learn that this is the case, they will not be as trusting of your statements in general, and it will be harder to maintain a good relationship with them.
- The real world is complicated enough to keep track of without the additional burden of remembering the details of the imaginary world that includes all of the little lies you tell. The more times you tell a lie, even "little white lies", the harder it is to keep all of your lies consistent with each other and the real world, and the more likely that you will slip up and reveal that some of your statements were lies, after which see the previous point.
- Even if you don't slip up to the point where someone else recognizes it, you may internally confuse the real world with your imaginary world that includes your lies and end up reaching conclusions or making decisions that are incorrect, inappropriate, or at the least suboptimal for the real world.
- Keeping track of your imaginary world and ensuring that you don't make the mistakes of the previous point uses up mental energy that could be used for more constructive pursuits.