With an LCD no. Some TV formats are subject to burn in (plasma, RPTV, etc.) so if you watch with black bars all the time then it could damage those tvs.
To clarify this a little. Black bars will not cause burn in on a plasma tv. They cause the areas of the TV to wear out at different rates. If you watch a lot of 4:3 picture, the middle of the TV will get a lot more use. As a precausion, most plasmas use gray bars instead of black and newer Dish receivers do this too. The gray bars activate those parts of the screen so they wear out more uniformly with the center.