i say 8 is too many. the more buttons you have, the more confusing it gets for your friends/family who try to use it. you will remember what every button is set to do, because you are passionate about it. if you put the start and select buttons right there with the action buttons for SNES games, you are going to constantly have to hit your friend for accidentally pausing the game because he keeps hitting the start button by mistake. like many others said, use the p1/p2 start buttons as Start, and the coin buttons as select.
speaking from experience, and many control panel revisions, more is not always better. the more controls and admin buttons you have, the more confusing it is for everyone else. i say keep it as streamlined and simple as possible. you are still going to have to explain how it works to your friends pretty frequently...they just wont remember how everything works. i have light-up, labeled Pause and Exit buttons, and people still ask me how to pause or exit the game.
on a side note, to those of you using dedicated mouse buttons for your trackball, there is no real reason you would need to do that. an easy way to clean up clutter on the control panel is to double up your buttons. wire up two of your regular player 1 buttons to the mouse, and just configure mame to use mouse buttons for those inputs. that way you will have access to the mouse buttons when you need them, but don't have to have dedicated buttons there for them.