So isu you're using mame before the video re-write, right? (heheh.) My experience is that in various versions, some things say like gamma make little or no difference, while some others depending on the version will screw up the overall contrast. Version-wise, this is how I have things set:
Mame .104 : full screen brightness - 1.50
Mame after re-write, in this case .131 : only the contrast correction is adjusted, to 1.35 .
Advancemame: believe it or not, I let it decide, and it automatically sets it at 33khz. While it isn't as defined as at a higher resolution (and may perhaps be similar to a beam width of about 1.5 or so), the phosphor glow is better because of the lower scan rate. Then, I set the vector luminosity to 3.0 . Very nice. This is on a 27" arcade multisync. I'd take pictures, but you might as well try it out yourselves.
[later] I went back and played again with mame .131 . Brightness just makes the display area all white, and full screen brightness makes the whole screen white, both at fairly low values, and neither really adding anything before making it hazy. Gamma does have some effect, where before the re-write it didn't. Contrast correction however has a much more dramatic effect, in which case any affect gamma had becomes negligible. Also, contrast correction beyond 1.35 there's little if any increase in 'effect'. (Full screen gamma and contrast do the same as gamma and contrast correction, but perhaps wouldn't in the case of a raster graphic game.)