Dont over think it to much

In reality you can use any button for anything on most things.
You set up emulators on a individual basis and configure the controls manually.
All you really need is a start and coin button for each player plus sticks and play buttons.
Any extras are personal preference. You can have exit buttons, volume buttons, tab button (which is to get into the settings menu on mame), service buttons etc etc etc.
You can also have hidden shifted keys where you hold down 2 buttons to choose something else.
For instance hold down start and button 1 you could send a F2 command to open a service menu in mame.
You just need to work out what you want to be able to do with individual buttons and go from there.