In a manufacturing company, is it better to apply serial numbers when
receiving a purchased item that will be included in a final assembly,
or as assembly is executed?

I am an ERP consultant. The answer depends on regulations regarging
purchased parts record keeping and that answer will lead tou to
formulate the best procedure for your record keeping needs and
especially the way your pDM software can track sub-assembly serial
numbers and how they may be related to yhe S/N for the whole SKU.

Suggest you do a search or dig into techrepublic.com and apics.org.

Regards,

Chad