By default, MAME assumes you have six buttons laid out like this:
123
456
If you wire them that way then most things will already be mapped correctly, including all the 6 button Capcom fighters.
The Midway fighters (Mortal Kombat series & War Gods) will need to be remapped.
If you want the 4 button Namco fighters (Tekken,

) to be true to the original 4 in a square layout, you'll have to remap them. Personally I prefer the way they fall with the default MAME mapping.
If you didn't lay out your buttons like that, then anything with more than 3 buttons is going to need remapped.