I settled on EMU, LIST, EXIT, PAUSE also volume and headphone jack. I used to have a shift button with joystick up down for vol, but I really didnt like it. What I assume is that EMU for most people will change what emulator is running... ie. MAME, Daphne etc... and LIST is to change the game list shown in each emu. In mame I have a lightgun list, trackball list, favs etc... I wasnt even going to put an EMU button, since I was only planning on haveing MAME, but now it seems like a good idea to add, just in case.
I would keep all destructive buttons off for sure, no tab. Kids could hit that and mess up settings, but they cant mess anything up with what Im putting on, and Im not useing any shifted functions. The only other thing I might add is save/load but that wouldnt be visible, id hide those somewhere. Maybe even a pop open hidden panel.