Actually I wear a white t or tank top underneath sometimes....Well you especially wanna wear a white tee underneath if you're sporting like khaki's and a belt to look formal. But I mainly wear a white T underneath for the convenience when I hit the gym or play sports with a few friends. Bottom line, a white tee or tank top underneath won't hurt. Plus if you get the polo dirty or stained you'll have something underneath.
Personally, I don't wear them...but, my friends who do, I've noticed are usually waring one of those white t's under it. I doubt it matters, but if I were to wear one, thats how I'd do it.
Just those crappy white t's...the ones you can see through. It doesn't matter, its an undershirt. Actually, MOST of the time I wear two shirts. Its just more comfy for me.
You should wear a white tee under the polo. Also, don't button it up at all. Look's better with an undershirt anyways. Trust me I know about clothes and what works lol.