It depends on a number of things. Generally, wearing it tucked in is a sign of control, wearing it out is more of a relaxed look. So you need to examine your surroundings and decide which would be more apropriate.
For example, while you are at work, you want people to see you and know that you are in control of your surroundings. That means dressing presentably, specifically having that shirt tail tucked in.
But if you are going to the bar, you are going to drink and unwind. keep that shirt pulled out; otherwise, people will think you are high-strung
T-shirts generally should never be tucked in, otherwise, you look like a poindexter. If you want to show off your belt, get a shirt that ends right at the belt-line.