I think it really depends on the context and the shirt. If the guy is dressed in a dress shirt or suite, it definitly needs to be tucked. On the other hand a polo shirt tucked into shorts could look really weird.
Tucked in looks smarter. It depends on the shirt, some dress shirts are shorter and not made to be tucked in, any shirt that you could tuck in, should be tucked in.. It will look crap otherwise. T shirts generally shouldn't be tucked in unless you're older or going for a preppy look.
Long or short sleeve tees I pair with slim fit khakis and tuck one or two "corners" of the shirt in so that it doesn't drape completely over my butt. I sometimes wear a plain black tee completely tucked into the same khakis but I can pull it off with my body type. I would not recommend most people try this.