I always always always wear an undershirt with a dress shirt, the crew neck t-shirt variety. I started my adult life as a Deputy District Attorney in Orange County, California, so the white undershirt beneath the maroon label Nordstrom pinpoint button down (white only, blue on Fridays when not in trial) was pretty much required by law. Given the amount of starch in those shirts, the undershirt was as much for my own epidural safety as it was for modesty. Lately many companies are making them in a trimmer fit with a bit of lycra, which helps prevent the fit problems of which Mr. Kabbaz spoke. But do wear a v-neck when not wearing a tie. Nothing says "UVA frat boy out on the town" like the white collar of a crew neck undershirt worn beneath a button down.