Bars aren't bad...
Blank letterbox bars on either side of (or above and below) the image are usually the result of the source material being a different shape than your television. Getting rid of them modifies the picture away from what the director intended and usually worsens image quality.
...but you can expand...
"I don't care about making the picture worse; just get rid of the bars." OK, fine. Most HDTVs and many high-def sources--cable or satellite boxes, digital tuners--give you options for expanding the picture to eliminate the bars.
...and fill the screen.
If you've expanded the image and don't mind the stretching, the cropping, the zooming in, or other issues that it introduces, rest assured that you're at least filling the screen with a moving image. This can help make sure your TV screen wears evenly over time.