What is the source of truth for invoicing in a Stripe and NetSuite setup?

This depends on your billing model. If billing originates in NetSuite and Stripe is the collection mechanism, NetSuite is the source of truth. If billing originates in Stripe, Stripe invoices or subscription events are the source of truth and sync to NetSuite for financial recording. Establishing this clearly before go-live is one of the most important design decisions in the project.