Your seat can do long-term musculoskeletal damage if not set up and sat in properly. An easy way to prevent injury, is to use a chair with adjustable seat back and arm rests and an adjustable height. While sitting in the chair, OSHA advises that your thighs should be parallel to the ground, your feet should rest flat on the floor and the seat front should not press against the back of your knees or lower legs.
Your posture at work can effect your health outside of the office. As much fun as it is to lean back in a chair to the extent that its wheels or legs leave the floor employees must resist the urge, or risk falling backward and hurting themselves seriously. It never hurts to take that extra second to check that the seat is beneath them before actually sitting down. The embarrassment may be worse than the pain.