I am in same boat, My Visa is Stamped till 17th March 2019 but my I-797 expired on 26th Dec 2017.I got My I-94 expiry date as 17th March 2019. I have filed my extension, I got receipt number but that receipt number is throwing Validation error in USCIS online portal (May be due to technical error not sure).
How many month it took to get approval of your extension. Does your I-94 expiry date was 28th Dec 2018 ? USCIS accepted your extension based on I-797 expiry date or Your I-94 Expiry date?
Yes, I think you need to go for stamping coz this new receipt number(Extension I-797) Should be printed in your Visa. Previously some old receipt number(your expired I-797) is there in your Visa which is already expired which may create problem at port of Entry while validation.