The regulations require processes or procedures to achieve a purpose, and sometimes explicitly include specific checks, authorities or record-keeping.
Your people should understand when in such procedures something must be done or when it is optional/preferred etc in any which way that works for your personnel.
If you use shall and should, and your people don't really care about that type of language-use you will have gained little to nothing.
No regulation or standards says you shall use shall and should (and must, and can, and may, and might, and will) in your procedures.