I either wear a white, grey, or black undershirt with my button ups and polo's. Depending on the shirt I wear a V neck or a regular necked. I always wear an undershirt though and the reason is simple. sweat stains ruin shirts. Would you rather ruin a undershirt or a nice shirt? I am not a heavy sweater by any means; however, before I started wearing undershirts I ruined a couple of my favorites by them getting a small sweat stain. I do not know if it was from the sweat or my deodorant. I also hand wash my polo's and button ups. Good ones are expensive.