I am no dvd cutting expert but I do have a serious fetish for quality output.
Always aim for hte top bit rate mentioned by marc, but if you must put more on a disc think carefully first.
If the video has lots of motion and fast cuts it will suffer far more than slow paced still life type video.
In the latter case exerimentaiotn has shown that you can get away with a much lower bit rate without too much 'postage stamping'. As low as half the max.