Q: How do I know if I should wear a button-down shirt tucked or untucked?
A: Length and fit will determine that. Below is an example of the right length for an untucked shirt. What you want is for the shirt to hit you no more than an inch below your hip crease. Proportionally, up to an inch and a half higher than the photo is good too but no lower or your legs will appear shorter than they are.