As someone who had to work on syncing multiple databases of customer and order data this was actually very important for me to know. Turned out that it could vary on a field by field basis and could also depend on the type of customer and where they came from.
To sync up our new and shiny SAP CRM with several Access databases and our customer-facing software I ended up writing a script that would collect all data field by field with varying hierarchies and writing it back out to everything. Worked surprisingly well.