Hi.
1. WIP custo is CO responsability. there are lot of info about wip custo
http://wiki.scn.sap.com/wiki/display/ERPFI/Understanding+Results+Analysis+for+WIP
2. Yep. From the financial point of view, is a must. Lots of consultant dont like to run WIP because is more work in the close but, if this is your bussines scenario nothing to do.
just for your general knowledge:
theory of wip>
http://www.inventoryops.com/articles/work_in_process.htm
SAP Help>
Work in Process in Product Cost by Order - Cost Object Controlling (CO-PC-OBJ) - SAP Library
Arturo.