This is a wonderful philosophy from Theodore Roosevelt, but if you stare at a computer screen all day you should keep your eyes level with the top 1/3 of your monitor. This will help prevent neck strain which usually occurs when looking upward or downward for too long. Most computer monitors can be adjusted to allow this. If the monitor is nonadjustable you can stack items under it, like the reams of paper shown in the picture below.
When sitting in an office chair always rest your feet on the floor and avoid letting your feet dangle. People tend to raise their chair higher than it should be so that they can reach the keyboard comfortably. If your feet are not supported on the ground you may experience numbness, tingling, or pain in your legs from loss of circulation. Shorter individuals may need to purchase a footrest or use something similar for support. In the photo below Matt has his feet wresting flat on the floor and his eye level at the top 1/3 of his monitor. Looks great Matt!!!