Nero 6 DVD authoring?

When I use mpeg2 files in Nero 6 (NeroVision Express 3 latest version) to author a dvd does Nero re-encode the mpeg2 files causing them to be of less quality than the original mpeg2 files that were created from my editing program? If so what do I need to do to get the best quality from Nero? Place AVI files created from my editing program in Nero???

Thanks for the help.

Express only reencodes files that are not mpeg 2 compatible,for example placing a divx file into express it will encode all of it,placing a mpeg2 file there it will just author it,dont forget that it also depends on the file size that you specify either 4.3gb or 9gb,anything that wont fit to the disc will also be reencoded