Im using 7 buttons per player - figured better to have them than to not !
My layout is <a href=http://homepage.ntlworld.com/andrew_ross/CPLayout.jpg>here</a> 4 players. The group of 8 buttons in the center is player start / credit, and the group of 4 in the top left is for MAME functions - ESC, Tab, ~, Pause.
Its a big JPG, (4235 x 1627, 338KB) thats what Visio saved it as. The Visio templates on this site were invaluable.
What I read here that influenced me, is to:
put your hands down in fists next to one another
rotate your right hand so its palm down
open fingers
put buttons where fingers are
That explains the irregular arc, but for me the buttons are very comfortable in use.
Just need to get that piece of Lexan on Monday, and I can finally finish it up !