It depends on what you do and what other information you are bringing from QuickBooks.
For example, you have:
- The shipping address
- The billing address
- The parent reference that you have the IDs for

If you do, then you can use those. Otherwise, you may want to import them first.

If you are just syncing the data with QuickBooks, in other words, you're opening up a connection to your QuickBooks data and then importing any new data or changed data into your database, then I would keep QuickBooks' ID and just assign your own autonumber to each person in your table, but store theirs as a reference. That way, every time you do an import, it just matches them up by the QuickBooks ID and then updates the record accordingly.

If you're working with a live copy to their ODBC source, then I would just say the best thing to do is just go with their ID. Otherwise, it's going to be crazy keeping track of all that.

You could set up a hybrid solution where in your database you literally have a table that's just the autonumber you assign to each customer and their QuickBooks ID, and that's all that's in that table. That way, when you connect to the customer through QuickBooks, you know what your autonumber is by referencing everything together. So your relationships would be: Your customer table, the cross-reference table, the QuickBooks ODBC table. But that's a lot of work.