http://www.llnl.gov/icc/sdd/img/guidelines.shtml
Know the Aspect Ratio You Are Using
NTSC has a 4:3 aspect ratio, your computer screen is something else, so converting screen video to NTSC is not 1:1. Either a region of the screen is converted one-for-one to NTSC, or the screen is pixel-averaged to the NTSC resolution. Test your computer's video scan converter to see how square objects are output. Depending on your system, they may appear rectangular. Also, video hardware and software use various image sizes. An image created with a 640 x 480 resolution, when transferred to a 720 x 486 system may result in elongated objects. There are professional applications which convert aspect ratios, resolutions, color depth and file types to their own internal values. You need to conform to a single aspect ratio especially when combining and compositing images from varying sources
Render to NTSC Resolution
Most likely, the resolution of your computer screen is much better than NTSC. For video, render images to NTSC resolution to save rendering time and memory space. Details at resolutions greater than NTSC will not be seen, and unfiltered high frequency energy can produce unwanted color artifacts. Computer video frame buffers vary in size between 512x480 to 720x486, so know the resolution of your buffer and render to that size.
(I have been looking for about the last ten minutes in how resolutions between tvs and computers relate, and it seems they are alittle different, thats why there appears to be a "uneven fit" in some cases. I use to study film and the standard film format when made for TV was 640x480, sometimes we had off framing cause of the conversion between the 29.97 FPS of TV and 24 FPS of movie film, to make it worse was the conversion between the film sizes, film is usually done at 16x24mm, thats the 16x9 widescreen, then the normal 4:3 format of TV... I suspect the same thing happens with computers, the aspect ratio is slightly different so it can vary.... but after 20 minutes of reading all I can say is, I get different information depending on where I go...)