To answer your question, there should be an opening balance for the account. This balance should match the balance in the account at that point in time (if you have the full history then the opening balance entry would be for 0.00). If it doesn't match or if there is no opening balance entry then yes you should enter it or adjust it to make it correct. Note that the opening balance entry uses a category of the account itself (a “transfer” entry back to the same account).