It’s not a crime to wear an undershirt as long as it’s not showing. We all know it’s important to protect your dress shirt from sweat and stains, but the undershirt is considered underwear so according to etiquette rules, it should absolutely not be visible. In addition to that, it adds unnecessary horizontal lines that ruin your body proportions. But if you really wish to unbutton your dress shirt, maybe opt for a V-neck shirt underneath instead.