stock maintain with hsn code

No but if the closing held by you on 30.06.2017 is eligible for ITC then you have to fill form GST TRAN-1 in which you have to specify the HSN codes of the stock lying with you.

Yes you have to maintain your stock with HSN code after introduction of gst regime so if you have any credit and you have to fill gst return then you have to specify the codes, otherwise not