When making titles and graphics for video, one must make sure that they conform to the standards of NTSC video. NTSC is much more restrictive than what computers can show, so graphics that may look fine on your computer monitor may flicker or appear washed out on TV. On the other side, graphics that conform to NTSC standards may look dark and dull on a computer monitor. It is best to work within these standards and then test your graphics out on a TV to see how they'll really look.
Graphics programs often have filters that allow only NTSC-safe colors, but these vary in quality. Some might permit some illegal colors to sneak through, while others, like the one in most Adobe products, can be too aggressive and reduce your color too much. For the most control and best results, it's best to take note of the colors you're using yourself.
Overly sharp edges can cause a problem for video as well. Since a scan line in video is represented by an analog waveform, it cannot instantaneously change from white to black; it needs some room to "ramp." When video attempts this instant change, it often overshoots, creating a ringing effect around the edge or causing the image to pulsate. Overly sharp edges can also create mosquito noise artifacts when compressed to DV or, in extreme cases when they are combined with illegal colors, the overmodulated picture signal leaks into the sound subcarrier and causes a buzzing noise.
The solution to the issue of overly sharp edges is to add either a horizontal motion blur of between .5-2.0 pixels, or a gaussian blur of .5-2.0 pixels. The will have the effect of antialiasing the image and eliminating the hard edges. If you are using vector graphics be sure to test them on a TV over firewire first to see if you will have these problems. I know it seems counter-intuitive to be blurring your images to make them look better, but pretty much nothing about NTSC is intuitive.
Additionally, there is another region that is inset 5% from the edges of the action-safe zone known as the "title-safe zone." Older picture tubes still produced some distortion outside this zone, making text harder to read, so titles were always including inside this zone. Most TVs nowadays don't have a problem with text outside of this zone, but it's still common practice to use it. You may want to play it safe, but if it's not possible I wouldn't worry about it, since the likelihood of someone viewing the video on an old TV that has this problem is low.
Video editing software such as Premiere often have an option where you can lay a diagram of the safe zones over your video for reference.
The way to make sure your images will look as you intend them to is to create your image at 720x540. When you are finished, change the image size to 720x480 and save this file (of course you should save the 720x540 file as well in case you want to make changes.) The 720x480 image will look stretched, but this is good. Premiere assumes that 720x480 images use non-square pixels, so when you import that 720x480 image into Premiere, it displays it as having non-square pixels, and thus it looks like when you created it at 720x540.
Another option is to create the image at 720x540 and simply import this. Premiere will automatically scale any image to fit the screen, so an image with a 4:3 aspect ratio will retain this ratio and look correct. However, Premiere's scaling is not the best and can sometimes yield results that are blockier than if you made your graphics the above method. 640x480 images will also be scaled properly since they also have a 4:3 aspect ratio, however quality-wise it is of course better to scale down than to scale up.