These days, there are a bunch of people (ie: hipsters) out there buttoning their shirts all the way up, giving the shirt an "invisible tie" look that I'm not much a fan of. Not only is it less comfortable than what you could be doing, but it just looks a bit off. My thought is, if you aren't wearing a tie, then at least unbutton one button.
On the other end of the spectrum, there are those who feel like it's okay to show off their bare pecs, and play strip tease with their business casual outfits by unbuttoning at least 3 buttons on a shirt. Cover yourself up sir! I'm a fan of the bench press as much as the next, but honestly, I don't like seeing other dudes' pecs and nips. Take it down a notch and unbutton just 2 buttons.
In summary, when you're wearing a button up shirt, unbutton 1 or 2 buttons (depending on how free you feel) and call it a day. It'll look good & feel comfortable. Booyah.