That's a difficult question, so many factors involved.
The short answer is 'do what looks best to you and still meets your bit rate needs'.
There're no firm rules I know of. I've seen excellent vids done at 120kbps and lousy ones at 9Mbps.
In general 2 pass average bitrate beats 1 pass constant bit rate. A motion vector detection setting of 20 or more is good too.
I've noticed the VP6 compression that China is going to use looks pretty good at low bit rates. It seems to distribute the compression distortion over a wider area on the video, avoiding the 'blocking' that MPEG creates.
Fav quote - "Experience is whatcha don't get 'till ya don't need it no more."
System - Athlon 1.4GHz, Win98, Hauppauge PVR250 receiver and compressor.
Software -Magix Movie Edit Pro 10, Nero 6 + NeroVision Express, Moho 4.61, PSP 8.1, Bryce, Quicktime 6.52 pro, Goldwave 5, DVD-Lab.
Cameras - Panasonic GS9, Canon ES8400V, Canon EOS D20 and Canon A70