I wear an undershirt and often leave the top button undone where it shows for three main reasons. One being I don't have a lot of chest hair, but enough that some hairs do show. Second is those V-Necks really just feel odd, and make me uncomfortable, which I know you (Antonio) have said in other cases to wear what makes you comfortable as it makes you more loose and confident (the tie/bow tie question comes to mind for that statement). Finally, I was in the military, and one of the uniforms I was the utilities. Blue button up shirt that you didn't button that top button, and a white shirt under it, and it worked. Undershirt also showed in working whites (button up again) and the dress outfits. The main thing you have to worry about is make sure the undershirt, part that shows, is crisp and fits against the form well. So I suggest you make sure you buy one that will be a bit tight, and make sure the collar isn't fraying or wrinkled. The tighter fit will help at least against wrinkles.