Measurement Uncertainty for 17025 Accreditation

As far as ISO 17025 requirements go, there are situations where a lab does not need to evaluate and estimate MU if a published standard method is followed.

That is if you are using a standard published method or a well-recognized test method  
and if the method specifies limits to the values of the major sources of uncertainty 
and if the method specifies the form of presentation of the calculated results 
and if the laboratory follows the test method without deviation  
and if the laboratory follows reporting instructions. i,e you control the risks

Even if the above criteria are met, besides ISO 17025 requirements you need to fulfill any client and regulatory requirements; and those specified by the accreditation body (these could be best practices enforced by a professional association or a particular program).

I suggest you confirm with your accreditation body and users of the test results. For example, a client may be part of an agricultural quality scheme that mandates the MU be below a limit for contaminants in animal feed; or there may be local regulations for fertilizers that specify a decision rule including MU (e.g result plus MU must be below the threshold limit to pass).