I don't think that there is a hard and fast rule on this. A lot of it is opinion ( and we all know what opinions are like). I don't let a batter come to the plate with a shirt flopping in the wind, I make them tuck them in because I don't want to make a call on a shirt being grazed by the ball. In high school ball there is a little less leeway they must tuck them in unless they are designed to be worn out ie: a different colored border.