Fixed pixel displays (Plasma, LCD, DLP, etc.) are said to look the best when fed signal at their native resolution. This is because in this situation the TV's scaler is not called on to do signal processing.
If your STB is capable of outputing video at your TV's native resolution, it might be worth the effort to see the difference. If not, it will be a hit or miss. It depends on if your TV has a good inbuilt scaler.