When is it appropriate? It depends where you are at and accepted protocol. Whether it is business or social, ask in advance what the expected dress is. If the setting is relaxed, your Shirt becomes the focus. Last thing you want to do is draw attention for the wrong reason. Make your shirt your asset, not a liability. When assembling your shirt wardrobe know what looks best on you.
Fit is dependent on body type, biceps, upper torso mass, muscles, size of shirt and pant waist. The ultimate goal is to acquire dress down fabrics to go with stretch jeans and modern pants. The fit should drape, not cling. Stretch shirt fabrics should look crisp, not flimsy. Have good body and weight. Shouldn’t be transparent to expose what is underneath.
Shirts worn untucked need to cover any exposed skin. Ideal length is 2 to 3″ below the belt. Make sure the fit compliments your body type. It should look as good if it is tucked in. Some of the stretch fabrics feature odor control, wick whisking, non iron and UPF protection. Better quality fabrics are 4-way stretch that are lightweight and look and fit great throughout the day. Many of the stretch are designed to worn tucked in or untucked.