You don’t have to do this at all unless you are migrating to Manager from another system. But I know you are not. Manager is perpetual, so there is no need to start fresh in a new year. See Manager Cloud.
Yes, I am. My main program is QB, for small companies and charities I am using Manager. I am considering moving to Manager and I was doing some small test work with it. Since 01/01/2018 I am about to mirror my QB work on Manager for few months and then take decision what to use further.
So, I am migrating (at least trying to do that).
Then you are in for some tedious work. There are two aspects to setting opening balances for Customers. If they simply have a monetary credit or you owe them money, you set that as an opening balance. But if there are open invoices, you have to enter them, with their actual dates (in advance of the start date) in order to properly load up Accouts receivable. See this Guide, if you have not: Manager Cloud.
Unless you need the exact history in Manager based on (I am about to mirror my QB work on Manager for few months) you will have that history in QB, then you could create one Customer in Manager called QB Unpaid Invoices and put in a single balance and then allocate those related receipts to that Customer, by the end of a few months that balance hopefully is near zero.
If you go with Manager then you could re-allocate those remaining as outstanding to their actual customer. You probably need a printout from QB to use as a tick off (control) sheet for those receipts.
That is one of the solutions. Other of what I think of is just enter sales only upon receiving the payment (not to invoice anything from 2017 at all). This will not impact sales of 2018 and for for VAT we use cash basis. That would be fine for the beginning, some sort of shortcut for this.
By saying mirroring, I mean doing double work for two-three months - one with QB and the same with Manger and see how it goes. QB is excellent on reporting and if I will be able to achieve the same or similar result without loosing much time with Manager, that will be fine.
You will have trouble with your second approach, @Solnce, because you will never be able to match closing balances from the previous system with opening balances for Manager. You will have Accounts receivable balances for customers in QB, but not in Manager.
It is better to keep this account (or open temporary account for entire amount) empty then process ~300 invoices just to have matching QB-M. As I said, this has no impact on sales 2018, therefore I have no interest in keeping old invoices on new system, but this has impact on VAT which is cash basis, so I will mimic cash sales and will maintain correct amount of tax.