I use the IPAC's shift key for all the admin buttons plus some. I like my control panel uncluttered. I kept the default 1-player start button as the shift key, but I'm considering changing it, to something like a pinball button, (as I have had player 1 press start and then player 2 press start, activating esc and thus ending the game, meh).
Press and hold player 1 start and then press:
player 2 start: esc (exit)
up: ~ (volume...)
down: p (pause)
left: tab (mame configuration menu)
right: enter
p1button1: F1 (hide cross hairs)
p1button2: F2 (service mode)
p1button3: F3 (reset)
p1button4: F10 (turbo button)
p1button5: F5 (save game state)
p1button6: F7 (load game state)
...and some others like F11, pgup, pgdown, 3, and 4
These not only serve in Mame as the function stated above, but help out in )p('s 3D-arcade frontend. (filter, search, menus, config...)
I also find them useful for other emulators and pc games.
It may seem overkill, but they're there if I need them, and if not, no harm done. Basically, I have double the buttons I mounted, and half of them are hidden to the common user. All they need to know is "press player 1 start and then player 2 start to exit the game". I'd like to make a instruction card though to make it idiot proof, and more realistic.