It is Mandatory to all registered person having turnover above 1.5 crore to provide HSN summary however for registered person upto 1.5 Cr provide HSN summary is made optional...
Notification 12/2017 CT , Dr 28/6/17 provides that a registered person having annual turnover in the preceding financial year less than Rs.1 .5 Crore is not required to mention the digits of HSN codes in a tax invoice issued by him. However, in GSTR 1 & 2 he must give the details of stock sold HSN code wise. So person having turnover up to Rs 1.5 crore may skip HSN codes