Hi,
In case of sales order, as long as you have standard price available in the material master, costing run is not mandatory. During PGI and Billing (if you have costing based CO-PA), only standard price is taken into the consideration to calculate the COGS.
But still it is client decision to go with existing standard price without costing run. Check with your CO consultant or post this in CO forum http://scn.sap.com/community/erp/financials/controlling
Regards,
Asik.