One thing to keep in mind, if you edit and add graphics to a Lettebox sequence, there is no going back to 16:9 cleanly, you have to up-rez if you need it. If you edit 16:9 it is very easy to Letterbox when mastering if that is requested.
Your DVD will want true 16:9 footage or else it will never look right on a widescreen TV.
DVD players can convert 16:9 to Letterbox for 4:3 TVs very well on the fly.
DVD player does not fit 4:3 Letterbox footage to a 16:9 TV at all, to get it to fit you have to use the TV’s zoom function and you are blowing up the image, not a pretty sight.