Since noone pointed it out, I will. An IPAC or a Keywiz keyboard encoder is equipped with a shift function. You can program the keywiz to make any keystroke you want with any combination of the buttons on the control panel. You can get away with no extra admin buttons whatsoeverJust wanted to make sure you were aware of this.
Now, to make your cabinet user friendly, a dedicated exit key would be a good idea. That way people can switch games easily. A pause button is a good idea, but I prefer this to be hidden if possible. It detracts from the arcade experience in my opinion. Unless you are running a limited list of games that you have personally set up and calibrated settings-wise, it would be a good idea to include a volum control of some sort. you could locate a physical speaker control somewhere, or use the volume control in mame. Different games have different sound levels. I play nba hangtime alot, and you have to turn the volume way up to hear it.
Other than those, I see no reason for other physical buttons on the cab. The rest should be shift functions that only you know. I had to reinstall a copy of mame32 one time because my wife accidentally accessed the shift commands on my desktop cp. She changed a ton of button and command assignments, and somehow managed to remove the ability to access the tab command menu. Don't let this happen to you.