I am finishing a music video and I wonder if I can just render it out at 720p and then change the pixel shape. I guess it depends on the compression of the first render. How compressed would you guys feel ok with? And into what codec?
No, "for display" means you intend someone to watch it rather than edit it.
You should just be able to render as widescreen DVD and everything will be taken care of for you. The only thing I'll say is that if you render 720 from a 720 project straight to widescreen DVD, you'll end up with black lines on both sides of the picture, so you'll probably want to switch the project over to NTSC (24p if appropriate) widescreen and crop all the footage to match the output aspect. Could probably do this with a nested project.