I got that info directly from the Premiere helpfile:

Using square-pixel footage for output to DV
To use square-pixel files in a D1 or DV project:
- If your final output is D1 (NTSC), create and save it at a 720 x 540 frame size.

I know, it sounds weird to me too But bringing footage in that's 720x480x1 pixel aspect ratio, and then saving it to a 720x480x0.9 pixel aspect ratio produces stretched images when I output it to DVD. 720x540x1 footage saved to 720x480x0.9 produces no stretching, but those weird artifacts.

- Neil