"mutual agreement" doesn't apply if an approach is applicable or appropriate. Maybe if you're doing a 60 year old property where the cost approach may not be applicable/appropriate the certification would CYA. I still say, if its appropriate in the valuation, you should still DO a cost approach and, at the very least, report the findings somewhere within the report. And, in keeping with accordance with No. 9 of the Appraiser's Certification, include the definition of a "complete appraisal".
PS - Ben, if you read #9 all the way through, it states ....(unless I have otherwise indicated in this report that the appraisal is a complete appraisal...)