From my side, in most implementation i have seen and used opeining balance posting date as 31st March(depends on client) and perform the closing. This is because if you post it on 1 April, than their might be some issues related to reporting if you run it for April. You have both the option, to select.
There is an awful lot “wrong” with the demo data, but as Pranav states you have different approaches, talk to the customer and see how the accountant wants to approach it, this is not really a system question it is a process and control question for the business.
Adam is right… We had chance to design demo data for navision long back and there we posted the opening on 1 April as we were not at all concerned for the performance/reports or otherwise. Talk to your client and make the final decision.