Personally I think leaving two buttons is good, unless it's for a more formal situation, where two buttons would be too "flirty".
Again it also depends on whether you wear undershirts/tshirts or not. If you do, two buttons woul be ok in any situation, just one would be kinda weird.
I've noticed that wearing undershirts is quite an American thing. I work with alot of American guys in London and they are the only ones that wear t-shirts under their work shirts.
Yup, Fash you are absolutely right! When I just went back to Paris from the States more than one year ago, I used to wear white t-shirts under my shirts. The cuties from my classes told me that it looked really gross: in Europe, only unsophisticated people or
NERDY ENGINEERING GUYS

would wear t-shirts under their shirts.
And it looks like most shirts you can buy in Europe are somewhat "thin". I mean by that if you wear a t-shirt or an undershirt, you would be able to notice it, not really aesthetic right?
So bottom line for the US: two buttons with undershirt, but make sure that you undershirt/t-shirt doesn't show through your shirt.