If the suit hangs well and is cut conservatively, it doesn't matter how many buttons it has. Sit down in it. If it looks dorky buttoned, unbutton it. Otherwise leave it alone. You should have a nice blouse under it anyway. If you want to be anal, press the skirt in the morning.
I'm an older premed and have been a manager for a few years. I have NEVER noticed whether the candidates I have interviewed for jobs have unbuttoned their jackets. If it's a nice suit, and it fits, that's all that counts.