When mirroring a sheetmetal part in SW2003 possibly earlier you can right click on the mirrored feature which is named OrigionalPart_Mirrored-> or something of the sort. RMB and select List External Refs. This is where you can select which configuration of the original part to use in the current configuration. (Don't ask me why they hid this here instead of being consistent with the rest of the software) This will then allow you to use exactly the flatpattern from the original part. Without having your drawing referencing the wrong part eliminating confusion. I think in 2004 they made this a bit more obvious but I haven't verified that yet.
Regards,
Corey Scheich