For some corporate buyers G7 certification is important - even if they don't understand what that means. The majority of print buyers would be clueless.
The important thing is that you, as a print supplier, have at least one appropriate print standard, with supporting specifications, that you can reliably adhere to and that your proofing is aligned to it.
Sustainable Printing Goes
Far Beyond Using FSC Certified
or Recycled Paper
This informative paper on deinking: demand, principles, problems and solutions also explains why printing technologies are not all equally compatible with paper recycling systems; and why just a small fraction of printed material in the paper can cause difficulties. Link To White Paper