Most LCD displays these days have square pixels.
You can check this on your own display.
Measure the width and the height of the viewable area of your screen, in inches.
Say you get 35 1/2" wide by 20" tall. Divide it like so; 35.5/20=1.775 - that's the physical aspect ratio of your screen.
If the native resolution of your panel is, say, 1920x1080, 1920/1080 = 1.777 - that's the aspect ratio of your screen resolution. 16/9 = 1.777 as well.
If the resolution and the physical size match up, the pixels are square. If they don't, you can figure out a corrective factor based on the difference between those numbers, but most everything is square now.
So, if your pixels turn out to be square, and you know the CRTs of the original games were 4:3 (1.333) then as long as your layout file for MAME sets the screen to be height = width * 1.333, your result should be correct.
To verify what you are getting is correct, measure the gamefield you are seeing on your screen in inches. Does height / width = 1.333? If it does, then that's right.