Since you used the term stretch I am assuming you mean take an ordinary standard 4:3 source and stretch it out on a wide screen tv (Hi Def) geared for 480, 720, or 1080 - yes? If that is what you were refering to I would have to say I prefer side box. Stretching distorts the view and makes things look unnatural - especially peoples heads IMHO. On older sets stretching can contribute to ghosting and other artifacts. If you have a wide screen dvd and play it on an older standard 4:3 tv that has the stretch function I would say the picture might not look that much different, but that depends on the tv. I would stick with the letterbox. Now if you send a standard signal through many Hi Def TV's they choke on it and the picture looks terrible if they dont have a built in scaler.

And if you are talking about rendering - well I would say render to suite the final user: little tv, big tv, computer, internet, big screen. You will have a quality product when they are matched properly.