I understand you can check the box 'Do not encode compliant files' to avoid any re-encoding of MPEG's.
Assuming the file is MPEG compliant, are there any situations where, even if this box is checked, the file is re-encoded?
if it's a compliant mpeg for the type of disc you are creating, then checking that "Do not convert..." option should tell DMF to use your mpeg video without re-encoding.
if the file is edited in someway (trim, enhance) the files will be re-encoded to make the changes permanent on the converted mpeg even if its compliant and the "do not convert" is checked.