I remember last year I think I needed to go to the amphur to get a kor ror 2 or something paper which is basically a copy of the marriage certificate entry. Is this still needed for marriage visa extension?

It depends on the immigraiton office, but many ask for it and it costs only 20 to 50 baht. If they asked you last year, they wil ask for it again.

Needed in Nonthaburi, which is under the Bangkok office.

Needed a new one for the past three years on Samui.

Agreed - get a fresh KR2 to show that you are still married.

It's needed at Chaeng Wattana AFAIK. You get it a few weeks before you apply for your extension. It shows that you're still legally married and not divorced and eligible for another extension.

Korat specifically asked for it, in addition to the flowery marriage certificate.