This actually applies to audits in general.
Why argue? 1) To avoid an unnecessary NC from my department on his/her audit report. 2) The auditor needs to understand the difference between "shall" and "should".
How to argue:
1. First I should take the "should" with respect and normally do verify my parts prior to shipment and record it. This is my basis of compliance.
2. Fundamentally, by definition, "should" does not obligate me to verify every part prior to shipment. In other words, based on risk analysis and business necessity, I can make deviations. This is my basis of argument.
3. Say, in a particular situation, if I verified my parts as usual I would miss an important shipping date leading to some significant consequences, and I held high confidence of my parts that they would pass, (and possibly with customer agreement,) the "should" procedure would allow me the possibility of deviation w/o non-compliance.
4. Of course, such deviation must be exceptional and decision making rationale must be recorded.
5. Under extreme situations I believe people would break the "shall" promise as well but "should" certainly gives more flexibility. If the process owner does not like such flexibility, use "shall".