I have a Pause and Exit button on mine, along with a dedicated shift button (called FUNCTION on my panel), plus I use the flipper buttons as shift buttons as well. I use Shift-Pause to get to the MAME menu, Shift-Exit to reset the game, Shift-P1 Start and Shift-P2 start for P3 and P4 start (I think only Track and Field, Hyper Sports, and the Trivial Pursuit games use these on my cab), and Shift-Player buttons do a variety of tasks like Service 1 through Service 4, Snapshot, Show Framerate (for troubleshooting), etc. The coin reject buttons are wired as my coin entry buttons. I use Left Flipper-Pause to save a game state and Right Flipper-Pause to load a game state.
I'm using a keyboard hack, not an IPac, so all the shifting is done through MAME.
As far as the button choices, they've all been selected to maximize the playing of Non-MAME games. Exit is of course ESC; Pause is P, which is pause in most games; P1 and P2's Button 1 are the Enter and Keypad Enter buttons; P2 Joy Up and P2 Jot Down are Y and N, for answering questions; P1 Joy are the arrows on the numeric keypad; P1 B2 is Space; and the flipper buttons are Left and Right Shift.
--Chris