Of course, every inspection must, at a minimum, substantially meet the requirements of the SOP. If an inspector fails to comply with the SOP, the customer would have valid claims against the inspector for breach of contract and misrepresentation.
Therefore, when in doubt about what the SOP requires in a particular situation, the inspector should err on the side of caution and exceed what the SOP requires. It is better to do a little more than what may be required than to do less and risk a potential claim and harm to your reputation.
A word of caution: if an inspector consistently goes far beyond what the SOP requires, a customer could theoretically argue that the inspector voluntarily assumed a duty greater than the contract required. Most inspection contracts contain language stating that the inspector will perform the inspection in accordance with InterNACHI’s SOP. An inspector who goes far beyond what the SOP requires may open himself up to a claim that there was an oral agreement that he was going to do a more rigorous inspection than what's required by the SOP, but this scenario is unlikely.
If an inspector voluntarily assumes a duty greater than the duty required by the contract, the inspector has an obligation to perform those additional tasks with reasonable care.