When to keep the truth to yourself

Ethics

I spend a lot of my time dealing with the public, and I have noticed a gap between the theory of honesty and how it actually works on the ground. In textbooks, lying is usually just "wrong." In real life, we mostly just balance being honest against being a jerk. For me, it usually comes down to utility. If a coworker has spinach in their teeth, I tell them because they can fix it. If they have a personality trait that is annoying but they cannot really change it in the next five minutes, telling them just creates a problem where there wasn't one. I am not talking about big lies or covering up crimes; I mean the small stuff. When the truth has no practical use but will definitely cause some emotional distress, what is the specific rule you use to decide if you should stay silent?