Our management decided to import monthly historical trial balances for 2017 for comparative reports. We have intercompany transactions. What is the best practice for intercompany AP and AR? I noticed that our implementation team posted them (2018) to Opening Balance account then created separate advanced intercompany JEs.

yeah you can't import journals without a related party into the intercompany AP/AR accounts. What we did was create adjacent accounts that could store GL movements without a customer or vendor and load all the historic movements to that. Then in the period before go live, journal them out to the P&L and create the proper transactions hitting the P&L account. Anything left over in that account gets coded to unrealised gain/loss, then you re-run your revals which should get you back to the correct opening balance in your balance sheet If you're just using a single GL account for AP and AR in the general ledger, then you can move it as a single transaction (depending on the currency mix in that account you might need a couple) If you need to know who owed money to whom at end of 2018, for example, then yes your implementation team is the right way to go, but allocating payments against invoices is going to be time consuming