The indvidual buttons should be as close as possible.... On a real arcade machine (6 button of course) they are VERY close together.  A common newb mistake i always see are button holes that are too far apart.....  When the buttons are installed there should be about 1/16-1/8 space between the lips of the two buttons.  And 1/8th is really pushing it.... it should be closer to 1/16th....  If you make them further apart it will still be playable, but it will "look wierd." and a neogeo span of 4 buttons is going to be hard to reach.