I always keep my picture format at pillar/letterbox. I tried all the settings to stretch the picture and it does not completely fill my screen either. There's a 1/4 of an inch from both left and right and just a little of top and bottom. Now, this does not affect me because I like to watch SD programs in 4x3. My DVI output on the Sony GW (input 7) does not allow to stretch the picture using the TV setting but if I use the component instead (input 7), my TV will allow it. It looks like the options under stretch will not change between letterbox, crop and squeeze.
If your TV setting allows you to stretch the picture, you will able to do it.