There is a setting in the mame.ini, "intoverscan", which will *add* overscan (I have no idea why you would want this), but you will need to lose aspect ratio to set it. Maybe that's what you're thinking of.
Coin-ops Next 2 package comes with artwork. If you use bezels (in mame), then the overscan should cut into the artwork rather than the game play area.
Some TVs allow you to adjust the overscan settings.