* Set your DVD player's picture-setup menu to "16:9" and never change it again.
* If the display auto-detects a 16:9 signal, you've nothing to worry about. If it's manual, select the 16:9 mode only when you're playing a 16:9-encoded DVD (the back cover will sometimes say "enhanced for widescreen TVs" or "anamorphic widescreen" or some such).
* On those increasingly rare occasions when you have a letterboxed-only disc (i.e., not 16:9-encoded but instead letterboxed on the disc) in the player, use the 4:3 mode.
Most often times, the DVD player will have a vertical zoom, so even on non-anamorphic widescreen movies, you can just use that intead of switching back to 4:3 mode on the TV. Of course true 4:3 dvd, you would have to switch the TV back to this mode.
Lately I have been doing the unthinkable.. lol
Just watching everything in 4:3... Even my HD scaled back to ED. This is a short term experiement of sorts, I fully expect to go back to using the native 16:9 mode.
But my results so far have interesting and mostly positive with this experiment. The experiment came about in a rather heated discussion with another HT enthusiast about screen aspect ratio formats.
I dare say, so far, I think I was wrong somewhat and he was right.
Non-Anamorphic scaling/zoom is not a very standard feature and you'll always need the DVD player in 4:3 mode for 4:3 content. Like Allan said, if things don't appear to be in the right proportions change the DVD player mode and TV mode to correct. For anamorphic DVDs you'll want the TV and DVD player in 16:9 to get the highest resolution picture.