Welcome aboard, TLiner.

Pause
Tab (for entering Mame setup)
Enter (for launching ROMS or making a selection in Mame setup or front end)
Esc (for stopping an emulator or exiting back to front end)
Del / Backspace (for clearing an input field in Mame setup) (not sure if I've actually seen this on a panel or something I just thought of)
Volume Up, Down, and Mute
Left, Center, & Right mouse buttons (typically located by track ball)
The FAQ has some thoughts on the subject.
https://web.archive.org/web/20160809113045/http://newwiki.arcadecontrols.com/index.php?title=FAQ#Number_of_admin_buttonsQuick rundown of the buttons you proposed from best to worst IMHO:
(This advice assumes that you are building a control panel on a dedicated cabinet, not a portable panel.)
ESC and Pause -- Good choices.
Enter -- Optional. You can configure your front end to use Player 1 Button 1 instead.
(joystick navigates the FE menus, P1B1 launches the game)
Volume Up, Down, and Mute -- Do you want buttons or a volume dial?
Left, Center, & Right mouse buttons -- Not needed. Consider using a cordless keyboard stashed inside your coin door instead.
Del / Backspace -- Umm . . . No. You'll understand when you configure MAME.
Tab -- For most setups, this is
strongly not recommended. (see the wiki link above) Once you're done configuring MAME, you will not need it. Consider using a cordless keyboard stashed inside your coin door instead.
Scott