Ask the MB -- Undershirts and Dress Shirts
A: Your papa taught you how to wipe your ass, shave your face, and tie your tie. Let's not start doubting him now.
Few things are more vulgar than a white crew neck tee under an unbuttoned woven shirt; you'll look fully unstylish and clueless. A v-neck isn't much better -- the lines look like ass and are the men's equivalent of panty lines -- but may be permissible in extremely cold conditions, either outside or in the office.
In conclusion, do what your dad says and leave the undershirt for the weekend.
(Note: Nothing we just said applies if you are Carson Kressley.)