Hello James,
For the derived books I usually see companies using separate accounts.
This makes the identification and reconciliation easier.
However, as you use posting layers, you can of course also use the same accounts that you use for the current layer.
Before implementing this in your live system do some test postings and run some financial reports for your accountants to familiarize them with the way how and to which accounts AX posts.
Otherwise, they might have difficulties doing their month end closing and reconciliation jobs.
Hope this helps.
Best regards,
Ludwig