DVD uses very specific video files, which is why we say not to change anything in that workflow. You must give Encore (or other DVD software) what it expects. The set-top DVD hardware only works with one kind of video source that meets DVD specs.
When creating clips for online/computer viewing, keep in mind that software video players typically do NOT respect the PAR of the clip. Premiere knows how to display widescreen clip with 1.2 PAR, as does the DVD player, but other players do not and typically will simply assume the clip is using square pixels (1.0 PAR) and will display accordingly, changing the intended shape of the displayed image.
Since we cannot rely on PAR to get the result we want, we must instead change the actual dimensions of the image, to come up with an equivalent end result using square pixels.
Ann had suggested 864x480 for widescreen SD video, and I don't disagree as the math works. Take your 1.2 PAR from the original footage, and do 720 * 1.2 = 864. We are creating a 16:9 image right? So 864/16 = 54. And 54 * 9 = 486.
So you have 864x486 and divide either side by 54 and you get 16:9 so that is the correct ratio using square pixels.
However, not to confuse things but if I go to Export Settings and select H.264 as the Format, under Preset I can find various presets for YouTube 480p SD Wide, FaceBook 480p SD Wide, iPad 480p Wide, and they all use854x480 rather than 864x486. I think probably because they want to keep 480 on the right, and they just fudge the numbers a little to get the other side of the equation as close as possible to 16:9?
What to do? If you have a Premiere Sequence that is DV Widescreen (720x480 with 1.2 PAR) and you want to export for upload/computer viewing and NOT DVD, then quite simply select an appropriate 480p wide preset and the work is done for you. Look at the presets mentioned above and you will see 854x480, with 1.0 PAR, and also Progressive - remember we never want to use interlaced video online, doesn't look good. Even if the source sequence is 480i (interlaced), don't worry, when you export using a progressive preset (480p output) it gets deinterlaced automatically. Just pick a preset and export and that's that, you don't change any settings before export.
Here's an example and the source is actually 1080i, but doesn't matter, it's the export settings we are concerned with. All marked in yellow, those are the defaults, I have changed nothing. Just selected the YT 480p SD Wide preset.
I should also mention that for exporting SD video as 4:3, with 1.0 PAR, the dimensions become 640x480 (see available presets) for online viewing. Again, the math is a little off since DV uses 0.9 PAR and so 720 * .9 = 648, but the industry chooses 640x480 and that is actually truly 4:3 if we divide both sides by 160. So basically, DV video is just a little weird with the frame sizes and PAR things....square pixel video is easier to deal with when doing math!!!
Hope this helps
Jeff