HI,
No valuation class assignment needed for service procurement in OBYC with WRX key.
Use t.code: OBYC, Enter Chart of Account, Double click WRX & enter your G/L account, keep valuation class BLANK & save.
Now try your transactions.
If needed different GR/IR clearing accounts for material,asset and service, refer a thread which already discussed:
http://scn.sap.com/thread/3154491
Regards,
Biju K