I would like to do a backup of some movies on DVD with File Factory.
In the output setting, by default the option 2 pass encoding is disabled. Do I need to enable it for better quality and does it worth?
Yes, 2-pass-encoding will give you better quality (for the same file size). Whether it is worth it is impossible to tell. The actual quality/size gain depends on the material to be encoded (and the encoding). You might save maybe 10%-30% (maintaining identical quality) for average material.
But to really know it, you can only try...
BTW, Wikipedia has a good explanation of the tradeoffs involved in multi-pass encoding: http://en.wikipedia.org/wiki/Variable_bitrate#Multi-pass_encoding_and_single-pass_encoding