What I would do is find the closest multiple of the vertical resolution and set it, and use the -nohws -sl parameters to disable hardware stretch and add scanlines.
So let's look at Pac-Man. MAME.DK sayds Pac-Man's vertical resolution is 288. 288 is close to 300, so let's double that for a vertical resolution of 600. We'll use 800x600 mode.
So in our mame\ini folder, create a pacman.ini. In that file, put:
scanlines 1
hwstretch 0
resolution 800x600
That should get you the old-school look with a nearly full screen. you can copy this ini for all the vertical ames with a vertical resolution of 288 (Galaga, Ms. Pac-man, etc.).
Now, Centipede's vertical resolution is 256. A multiple of 3 gives us a vertical resolution of 1024x768, so we'll use that resolution and Centipede is now full screen!
scanlines 1
hwstretch 0
resolution 1024x768
Nw Tron gets tricky. The vertical resolution is 512. A multiplier of 2 gets us 1024, for a resolution of 1280x1024. The higher the resolution, though, the more horsepower MAME needs, and not all monitors are going to support resolutions this high. If your monitor does, though:
scanlines 1
hwstretch 0
resolution 1280x1024
At this very high resolution, the scanline effect will be so subtle as to be almost unnoticeable, but Tron will be running full-screen.
--Chris