Sync the minimum that both systems need to act on, and make one of them the owner of each field. Marketing-only scoring internals should stay in the marketing platform; the CRM needs the qualification state, the source fields, and the timestamps. Bidirectional sync on a field with two owners will oscillate.
Write the field map as a deliverable with an owner column before anything is switched on — it is a hard gate, not documentation. The usual failure is a marketing-platform custom field that has no CRM counterpart, so a workflow silently stops firing after go-live and nobody notices for a quarter.
Every claim above comes from work LeanScale published. These are the sources.